So sorry to read about this OP. Sadly there is a well worn script that they follow during these times, delete, deny, minimise.
They delete the evidence, deny it happened, then when shown the evidence, attempt to minimise what happened....
So it moves from chatting, to chatting and texting, to pictures and web cam, to meeting once, meeting more than once, only kissing, only a blow job etc etc.....
Do you see.... They will only admit the minimum they think they can get away with.
I think you need to brace yourself for more revelations. If they'd met online and lived miles apart, I could understand the pics well not understand bykwim
The fact that they know each other in RL would make me more suspicious.
Has he been acting any differently lately...? Attached to his phone, working late, working extra day at weekend, nights working away...?
All these are red flags that I learnt through experience 
If it were me I'd be checking Internet history, email accounts, deleted text messages and bank statements
although I'm the sort that needs answers !!!
At the least I'd ask him to leave for a few days so that he begins to get how seriously you view this.
Here's a
as its past 6 