Definitely some nasty work! I expect any man to be emotional and pissed off along with being embarrassed catching their woman getting that sausage out in public, but at the same time, what's arguing gonna solve?
Obviously, the to be ex GF(she better be a future ex!!) don't care nor respect dude, so it is much wiser to walk away and deal with his emotions than continue to put himself in a situation where he, his cheating ex and the other dude could get hurt.
As for the cheating ex, she need to be careful and be a lot more discreet in the future. That's how ppl get unalived out here in these street playing with other ppls emotions.