dump her
You go rape her, and fist her.
If you think you are in love with another person then you should break up with your girlfriend, because that's not fair to her or you.
Get a boyfriend. Then just tell your girlfriend you found someone else.
No.
You should just ignore them and date someone else better then her or him.
You should get to know her and see if you still like her and if you do, then you should ask her on a date before someone else does.
Leave them alone together. Find someone else to like.
this is bad! you should not be with her if this is the case!!! talk to her and say 'who do you prefer, him or me? ' if the answer is you then tell her you love her etc but if it it someone else then say 'it's over!!!'
If he is with his girlfriend and she did not bring a friend for you, no. If he has a girlfriend but is trying to hookup with someone else, yes, however you should have priority if you're single.
Ask her.
Be honest with your girlfriend. I have a friend who has a girlfriend but he is in love with his best friend, the longer you leave it then the harder it will get because your feelings for this 'someone else' will not disappear. Depending on how long you have been with your girlfriend it is the time to think whether you will later regret your actions or have any feelings for your current girlfriend. But in this case honesty is the best policy because no girl would or should be lied to if you are in love with someone else then tell your girl!