Ways to get rid of mum's boyfriend?

My MC is an angry and disillusioned teenager who has just met her mum for the first time since she was a baby. Her mum is blissfully happy, living with her boyfriend of many years, seemingly completely uncaring about the fact she abandoned her daughter all those years ago. My MC decides that to get a bit of revenge on her mum, she will get rid of said boyfriend, so her mum will know what it's like to be abandoned.
My question is: how does she break them up? Can be as nasty/malicious as needed, to be honest I'm thinking the more twisted the better for this character, but all suggestions are welcome!
Thanks!

Well, I'm sure there's all kinds of ways she could be really awful to him, especially if she is living with them. Taking, hiding or breaking his stuff. Making sure his portion always gets burned when she cooks supper. Sabotaging his razor, shaving cream, shampoo, etc.

If she wants to actively sabotage their relationship, rather than just make him miserable, she could try and convince him that her mum is cheating on him. Probably not by telling him outright, because that would make it too easy to detect the sabotage. But she could plant some kind of "evidence" of the affair around the house when he's not home. Maybe a tie or a cuff link left laying around that he would know isn't his. Make suspicious credit card purchases, if she has access to her mum's card. Have flowers delivered at a time she knows he will be home, either with no card or with a card with a faintly suspicious message. Of course the mum will claim to have no knowledge, but it might be enough to plant the suggestion in his mind.

Or she could go the other way, and make really nice with the boyfriend. Like, call him daddy and want him to do father-daughter type stuff with her. If he's got a problem with comitment, I'd say he'd be out the door pretty quick. If that doesn't work, she might step it up even farther and try to seduce him. With the intent of getting him to leave her mother for her and then dumping him, or of claiming he was the one making the advances and getting him arrested.

After integrating into their house as the daughter figure, use physical access to the boyfriend's work area or whatever to plant false evidence that her mum is cheating on the boyfriend. Manipulate them both. For example, she could send a letter to the house she knows the boyfriend will read, saying,"Meet you at 8 o'clock. Let's go to the cinema." and then send a letter to the house she knows the mum will read, luring her to some faraway place at 8 o'clock. When the mum is gone, say things like,"I wonder why she's out so late." The boyfriend will start to wonder. Plant false text messages on the mum's phone. Steal her phone and change her contact name from "Daughter" to "Boyfriend", then use this new "Boyfriend" status to send love letters to the phone. Make sure the boyfriend reads them.

Either seduce the boyfriend, or make the boyfriend think the mom is cheating so that he leaves

Convince the boyfriend that the mom is doing something bad. If he's rich, make it look like she's only in it for the money. If there's something that's really important to him, ruin it and imply it was the mom.

Say if she's dogsitting for him, sneak in and do X to the dog (where X is anything from dump maple syrup on it to shooting it, depending on how evil your character is.) The mom will deny that she did anything, but who else could it be? Fights can snowball. It doesn't have to be a dog. Something that belongs to him can go missing, or be destroyed, or something he trusted her with can get lost. If he's in a competitive business, information he told only her was leaked to the competition, and his enemy's number is in the mom's phone.

If the boyfriend has any self-confidence issues or major vulnerabilities, take a letter or an email or a recording of a phone conversation out of context and make it look like she's complaining about him behind his back, or telling his secrets or even planning to leave him.

She can also make it look like her mom is lying about little things. This wouldn't work on its own, but it'd be a great way to plant suspicion.

Boyfriend: What did you do today, honey?
Mother: Oh, I took my angry and disillusioned teenage daughter for ice cream.
Boyfriend: That's nice. (Goes into the kitchen, sees MC scooping ice cream.) Didn't you just have some?
MC: What are you talking about?
Boyfriend: Your mom took you for ice cream, didn't she?
MC: No. I was with my dad all day. I thought Mom was with you.
Boyfriend: SUSPICION!

Or she can dig up dirt on the boyfriend, things he'd tell his girlfriend of many years but not want generally known, and then casually drop it into conversation like her mom told her.

Oh, these have all made me smile so much! Brilliant suggestions, thanks folks!

I say sleep with him.

Or, well, hmm... Have you seen Desperate Housewives? Somewhere in season 2, one of the main-characters' son is pretty much trying to do the same thing to his mom's boyfriend. He's very subtle about the whole thing, saying some really offensive things to the boyfriend the moment before the mom enters the room - making it seem like the boyfriend's a horrible guy who gets pissed at the son for no reason whatsoever when she comes in and sees him yelling. Maybe something like that?

No one's going for the backstory here- the mother abandoned her baby! If the boyfriend didn't know about this before the daughter showed up this gives the girl ample ammo. Just moan about it and make it the mother a horrible villan from the daughter's point of view. And the guy starts rethinking his situation and who he's hooked up with. It doesn't even have to be deliberate on the daughter's part- she's a teenage girl and will of course angst on whatever's screwball in her life.

Truly mallicious? A false story the mum can't disprove, and the daughter has overheard the boyfriend saying that he truely despises a certain action /motive.

I don't know if it would fit into your story, but I just looked at the list of threads, and right next to this one was, "Hiring faery mercenaries! How to do it?"

Interesting juxtaposition.

Plant porn in boyfriend's stuff. Extra points if it's child porn (which is illegal to possess in most places, so getting hold of it could be interesting). Double extra points if kinky equipment or kidnapping items are added, and major bonus if the girl contrives to get all of this publicized. Fastest way to permanently ruin a man's reputation, guaranteed.

This is a bit risky, in the sense that Mom may stand by her boyfriend and deny he did any wrong, no matter what. (Especially since she's right!) But it'll definitely put a huge load of stress on both of them, and could lead to a lot of fights or at least make them both miserable.