Act like how you normally act. If you're goofy and fun, act like that. She'll love it. (In a friendly way, that is.) If you're not so nice (I hope not), then act nice and likeable and then try being that nice person. She's already taken, but that doesn't mean she can't have friends, right? :)
If you want to act in her best interest you should just treat her as a friend. If you want to get with her then you should do the complete opposite
Don't disrespect her relationship.
if she already taken then dont bother trying to get with her
guy are stupid -_- no u could be her friend
Her best interest is obviously you! Just slap some sense into her...