The thing is Mikeymum27 is that he will lie. The OP will not get the truth by sitting down and having a talk with him.
The stag do is the kind of far fetched shite he may come out with. Men go to lap dancing clubs on a stag do sometimes (and I'm not defending that!) but they don't book prostitutes for it!
Say that was the god's honest truth "I was looking up a prostitute for a friend who is about TO GET MARRIED". Well. That's reason enough not to be with a man, tbh.
There's just no point in you encouraging her to minimise it, because he'll do quite enough of that.
What does the OP know?
- Their relationship has been bad
- He's become secretive with his phone
- He has looked up prostitutes
I think the OP should ask him to leave, and if she wants to discuss it at all, do it during counselling sessions.
Because all she's going to hear from him is "I was just looking". And decent men don't just look.
Many usual declaration of bias: XH was a habitual prostitute user "I only looked" and is currently doing the same to his new girlfriend too.
It's depressing common. And not in any way acceptable.
I'm sorry OP. Tell him you know, and be prepared for the script of lies 
