I'm going to lie to my bestfriends girlfriend
Why would you lie to her?
I love my friend I don't think she is good for him.
That is going to hurt your trust with him. You should just be honest and not hurt anyone's feelings.
I can't help it I just have to be who I am.
Sometimes we have to be adults and respect our friends even if we don't like it. If you give him an ultimatum he might pick her over you.