I'd say it's probably more his friend keeping it from him that hurt, more than them actually dating. Also if she cheated on him a lot maybe he's worried that she'll do the same to his friend, or is feeling insecure and thinking "why did she cheat on me but not him, what does he have that I don't, why can she be faithful to him and not me?" It's probably making him doubt himself.
I think you probably need to give him time to come to terms with it and for the time being maybe it's best to let him distance himself from this friend, but don't let it happen forever. Maybe suggest them talking in a few weeks or so and just be really supportive. I wouldn't take it personally, like I said it's probably him doubting himself.
Really think your best bet is to let him get his head round it, and maybe ask your friends why they felt the need to be so dishonest, what did they think it would achieve? And try not to let any more friendships be permanently ruined.