I caught my fiance talking yo his ex once. I think it depends what he was saying to her. He hadn't deleted any of the messages and let me read them all and it was completely harmless conversation just small talk. But it still made me uncomfortable so he doesn't talk to her anymore. It's definitely OK to be mad if it was inappropriate but I'd try to talk to him first and let him know it's not OK that's all it takes some time

Well I say cheating is cheating. If you have to delete a msg you hiding something.

I would be completely mad. that's, an ( ex ) not a friend or nun someone he use to have sex with kiss go with etc. that's disrespectful and he wouldn't hear the end of it.