you should do nothing...just be patient and wait for the right time and you will know when the right time is.
You date her best friend
if they are your best friend they should understand
just tell him and get over it you should feel happy for her she is your best friend
you should try asking your boyfriend to talk to him
Talk to her about it.
no
I think you should ditch your best friend and replace her with another person who won't betray your trust by dating your boyfriend. That's what you should do.
You should tell her, they should be ok with it because they are not in love with their cousin and plus if she is really your best friend it will be cool with her.
You should talk to your friend who is dating the other friend's ex about telling her. If she refuses and continues to see the ex, you might want to evaluate her interpretation of friendship and what kind of friend she is. Based on that, you may want to tell the friend yourself if the other refuses.
well if you like your best friend then you should go out with him|her... but if you don't your best friend then you should tell him|her, that you are going out with someone already.
Neither of you should get to date him.
Have the talk with your friend about dating in the hypothetical sense. Or have another friend find out for you.