I'm not going to beat anyone's ass, I'm not going to flip out on the chick, I would just leave...fuck the both of them
It actually gains you your dignity in a no-win situation, because when she goes back to the guy and tells him you told her to fuck off, even though he "got the girl" he's going to be like "fuck that dude is legit", and the chick is going to be thinking "damn, that fucking dude rolled out on me"
if you take her home and shit, that will be laughed about when he plunges his cock into her..."what did he say when you said that ?? hahahahaaa, really? ...yeah lick my balls just like that"
And if you fight him, you look like you gave a shit...and who the fuck is fighting over a girl on a first date ?