I'm going to catfish my friend
Why would you catfish your friend?
I want to steal her boyfriend
Catfishing is mean under any circumstance. But to do it to a friend, and so you can steal their boyfriend, is really rotten. You're going to lose a lot of friends if you're willing to do something like that to them.
What's the big deal, I'm just giving her a taste of her own medicine.
I understand you are mad at her, but this is not the way to do it. If she is that bad, stop being friends with her, but don't catfish her and steal her boyfriend that is reprehensible.