I wouldn't get involved with this guy until he's figured out for sure what he wants to do in the relationship he's in. If he wants to break up with her then he should break up with her without having someone to fall back on. It's sucks enough to be left it sucks way worse to be left for someone else.
If he's only staying with her for financial reasons then he needs to find a way that he can be financially independant or he's just going to move from being supported by her to being supported by you. I would want anyone I was with to be with me because they want to be not because they have to be. Down the line you might find yourself questioning if he's just with you because he can't afford not to be.
Hold off on getting romantically involved with this guy until he gets some of the stuff in his own life sorted out.
Everything will be ok in the end. If it's not ok it's not the end.