It doesn't matter what I wear around a guy if he cheats on me, he wouldn't be my boyfriend anymore at that point so his opinion wouldn't matter. Dressing sexy in this situation makes it look like you're trying too hard, dressing like a bum makes you look depressed and like he effectively took control over your emotions; neither of which are good.
why would u bother getting his attention after that? i personally wouldn't feel the need to impress an asshole and if im gonna dress sexy it'd be for the next guy that comes along. plus if u dress like a bum he'll be glad he left u :o