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if he is really into that girl then back off but still be strong friends. if that girl and him break up then after he recovers ask him out
try any book by Sarah Dessen.
Because they still like the girl they are dating, but are bored. Or because they suck and they are a player.
be nice
Yes, but it is an open door to let's hook up again, or your new girl friend or boy friend doesn't really like me.
it depends if you like her do not break up with her if you don't leave her
Staring would be on sign the she might like you but there is other things like texting you a ton or asking your friends to spy on you.
Say "You should break up with that girl she secretly doesn't like you at all and is going to break up with you later on".
You should wait till they break up.
You're not dating so you don't have to break up.
move on
there is nothing you can do except if you still care about her try to make it work