I'm going to help my best friend to cheat on her boyfriend
Why would she need you to help her? Are you sure you want to butt into other people's business?
My friend told me her boyfriend is a jerk who mistreats her. I choose to trust her
You should stay out of their relationship. Cheating on someone is wrong you should convince her to break up before cheating.
I don't think I can convince her to break up and also I just want her to be happy, I think cheating will make her happy.
What you think doesn't mean it's best for her. You might actually end up doing more harm than good because cheating itself is immoral and never ends up being good down the road. I think you should help her in other ways, like going on a vacation to clear her mind.