Hi there
I feel very sorry for you,especially as I have been there myself. I don,t wan t to hijack your thread to repeat my 'story' and I don't know how to do links but I suggest you search for my earlier posts to see some of the good advice I was given.
What I would say is that there is some good news here (believe it or not) to suggest that you may have caught this at an early stage.
It costs a lot of money to visit escorts, and if your dh is volunteering to let you look at the bank accounts, then it suggests to me that there is nothing to hide. When I finally realised what was going on, there was a clear trail over years of him withdrawing £200 every so often, or sums over a few days to add up to £200 or so.
I also think it is a good sign that he admitted to the adult friendfinder thing of his own volition, without you having to confront him with your evidence.
On the other hand, I do think that the evidence would suggest that at some point he has visited a prostitute. The first time I found out that my h had looked at escorts' websites, he told me that he was 'only looking' and had rung one up just as a fantasy. I believed him totally, but it was complete bollocks.
Of course, for you, the fact that it has happened once may be a deal breaker, and who could possibly blame you? But if you feel differently and think there could still be a chance for your marriage then the fact you have discovered this before it has turned into a full blown obsession could be a good thing.
On the STD thing, then I do advise that you, or at least your dh gets tested. My h's tests came back clear and the GUM clinic said that there was no need for me to do it too. But you may well wish to be absolutely sure and want to get tested yourslef too.
I also advise going for a smear test even if you are not due for one. I didn't realise at all that condoms do not protect fully against the HPV virus, which many prostitutes carry and which causes cervical cancer. I was furious with my h for allowing me to think for years that I was low risk for HPV, when this was categorically not the case.
These are just the practical things. The emotional impact of finding out something like this is just awful. Look after yourself; realise that this is absolutely not about you or any deficiency on your part. It is totally about him and his failure to behave decently.
As for telling C's wife, it is very difficult. Of course, I wish that someone had told me what my h was doing. What I would say is that if the idea of damaging the friendship between your dh and C is holding you back, then I wouldn't let that stop you telling her. And it might well be good for you to have some real life solidarity and support. But as you say, I think you probably have enough on your plate at the moment.
By the way, you are not an idiot wife. He is the idiot - putting everything at risk for some kind of puerile thrill seeking.
I will be thinking of you. You are not alone in this.