I would so he can at least know so he can think about it
NO, don't be a homewrecker.
Trust.
Leave them alone together. Find someone else to like.
Calmly remind him of the fact she is already with someone, and ask him to stop.
freindship means friend
*you respect your friend by not going after HIS girlfriend, even if you like her very much ,friendship is more important don't you think. so i guess the best thing for you is to move on, to respect your freindship with your friend. i hope i have helped you.. =) you could just be like dude i like your girlfriend
No he already has a girl friend
hang out with her more often and then after a few days, ask her to be your girlfriend
no because then your girlfriend would be cheating on you with your friend and you will be mad
You can try telling his/her parents.
If that friend means more to you than your girlfriend, ask your friend how she feels first and if she doesn't feel the same way then stick with the person you are already with.
I'd ask your girlfriend's friend that question.