support them both
First dump her best friend. You're not doing anything to advance on the girl if you're dating her best friend. Next, tell your best friend how you feel. Then tell the girl that you still like her. If she wants to be with you, she will make the choice to break up with your best friend.
Say to your bf: " Look we have been dating. I get it shes your best friend but she should stay your best friend and I can stay your girl friend. I really like you! Please don't break my heart."
if they are your best friend they should understand
Then don't be his friend anymore.
apoligize
If she one day decides to break up with your friend, I see no reason why you wouldn't be able to ask her out.
Break friendship.
Your best friend. Usually teenagers have a best friend who will know how to deal with breakups and relationships
you just do it. mmk.
leave that situation alone, that's your best friend do not let a girl come between you two and if she was real about her intentions, she'll leave it alone to. If this personis truly your friend then you should not date his ex-girlfriends. If you still want to and if his girlfriend told she is going to break up with him and wants to date you then you should first ask your friend if it is alright with you dating her. If she will break up with him to date someone else she could possibly do that to you. If your friend is not alright with you dating her and you do and she breaks up with you, then you are left without a girlfriend and best friend.
Don't tell anybody, laugh it off. Try to win her before him. Or if they're dating (your friend and this girl) wait until they break up.
wait for them to break up and then see if she's interested in you.