I think the best approach is to say nothing.
If he isn't sure if he has feelings for her yet, then there is no reason to tell her he might have feelings for her, since what can that accomplish? It's even worse since she's in a relationship.
If he knows he has feelings for her and what he is contemplating is asking her out, then I'd still recommend saying nothing. Perhaps, if she has only been on one or two dates with the other guy, then asking her out might be okay, but anything more serious than that, definitely not.
In general, the first question is what does he hope to accomplish? I can't think how anything good comes of this.