If someone I am with loves another person then what the heck is the point? If he is so in love with another person they can't have they clearly need to get their priorities straights and get over it before trying to enter another relationship. If I had this knowledge I'd wish them good luck and refuse entering a relationship with them.
No, if he's in love with someone else then he should be with her, not me. I will never settle for "second best" and if I find out a guy is just settling for me because he can't be with someone else, I would no longer want to be with him.