You have 2 problems here.
Your first problem is that your dh's jealousy ruined your friendship. I have plenty of male friends and one in particular who is one of my best friends. I can happily go out alone with this friend and my dh would be fine with it. You know why? Trust. It appears that your dh did not trust you. Your word that you were not having an affair was not good enough for him. He forced your hand by telling everyone that you were cheating on him, thus making it impossible for you to continue your friendship.
This in itself is out of order. Who tells everyone that their partner is having an affair without any proof? Surely the first thing you do is to sit down with your partner and discuss your worries with them? Did he give you the chance to compromise at the time?
I find his behaviour there very controlling.
The websites are your second problem. Yes they were last year and the good news is that he hasn't logged on since then or answered your friend's message. However, he hasn't deleted them either has he?
As far as I am aware, men have to pay to join these married dating sites whilst women can join for free. So last summer his intentions were very clear, he was willing to pay to join a site where he could possible engage in no-strings sex.
There have been numerous articles about these sites and it's all about secrecy, discretion, having sex with other married people and not getting emotionally involved.
I think you do need to sit down and talk to him. Ask him why he didn't delete his accounts. Why did he feel the need to set them up in the first place? He needs to be honest with you about what was going on then or you won't get anywhere.
You have no proof that he has done anything but you have all the proof you need that he was going to do something and perhaps, if you hadn't found him out, perhaps he would have done. The thought is still there as the accounts are still open.
Don't allow him to throw your friend into the mix. He knew about your friend when he met you. There was no secrecy there. You were open and honest about your friendship. He decided he didn't like it and he forced your hand. That was a low-life and dirty trick to play. How would he like it if you told all his friends and family about his dirty little profiles on these websites? After all, what's good for the goose......
Honesty is the only way forward. If he can't be honest with you, if he turns it back round to you, if he denies everything and won't discuss it further then I fear for the future of your relationship. It seems that when things are going well, he's happy but the minute something goes wrong, well then those accounts are there for him.
I'm sorry but I would seriously question my relationship as I would feel that I didn't know my partner as well as I thought. The trust would be gone and I'd be paranoid that after every row, he'd be seeking a quick shag elsewhere.