It a slang term the regular posters in this section of the forum (and on some outher parts of the web) have for someone who likes in any way someone of the opposite gender when they are in a relationship.
I think it's B*llsh*t and so do quite a few others I've seen.

On topic-My advice to you is best summed up by an old cliche, 'All's fair in love and war'

That's like saying 'If he's willing to break up with her he's willing to break up with you' or 'If he's willing to divorce her he's willing to divorce you'
In some case probably true, but in most cases apples and oranges.