My bestfriend asked me to cheat on her boyfriend with her.
Is she interested in you?
I don't think she wants to sleep with me. She wants to hurt her boyfriend in this way.
I think you should stay out of this. Their relationship does not have anything to do with you, there is no reason to get involved.
She was just doing it to hurt her boyfriend so I'm not going to get involved in their relationship and I don't think I'm doing anything wrong.
It is good to not get in the way. Even joking about cheating is enough to be unfaithful in some eyes. Tell her to leave you out of it.