I don't think she has a clue that he's a player and I would really like to warn her, but should I?
Most Helpful Guy
Of course you should. If you saw someone approaching a hidden landmine, would you tell them to stop, or let them be blown to bits?
Lack of communication is why players are able to manipulate people. don't just tell her, tell EVERYBODY, especially other girls!
Most Helpful Girl
I'd say yes and no.
It would be very nice of you to warn girl #1, because NO ONE deserves to be played on. BUT If you really want to warn her, try doing it anonymously, like sending a note, trough other people etc, if possible, because otherwise she might think you are just jealous he is not longer with you and can even say something mean to you.