Exactly where were these notes???...Could it be that maybe his guilt "wants" him to get caught???...any indication if this is someone you know or is it a one night stand...or even out of town???
I can caution you on one thing...don't ask for ANY details...I had a counselor caution me about this...while my imagination went wild I think this was good for US initially...eventually...like about a year after he was home I did ask some questions...H said he didn't really want to talk about it but if I needed it for closure he would ablige...somethings I imagined much worse...some things not...but I am sure had I known these things earlier it would have hindered my ability to forgive...
I would just wait until you see how the counseling goes...see what H wants to talk about...maybe inbetween times you can see the counselor on your own and ask how best to handle this...maybe C could bring it up...maybe offer an "amnesty" night...H can say anything he wants and you promise not to lose your temper or get angry...also, you promise to work on forgiveness if it is needed...or wanted!
I did the amnesty thing with my H in the past when we were having problems and I knew things were going on online...EA...it worked for healing then...and I think at the time I would have been prepared to handle it if there had been a PA...when the PA actually happened I was totally caught off guard!