Pelagia21 I really feel for you having been through something similar albeit in a 'milder' way. It is very similar here with my OH (we are not married) striking up a friendship with a new colleague, 26 year old girl, last year (he is 41). He works in a very female led environment and I'd never had an issue at all with him being friends with women at work over the past 10 years of him working there, but this seemed different and I think I intervened/found out before it escalated into some kind of emotional affair. My OH is absolutely blind to it all and was adamant nothing was going on, but I do think this girl was crushing on him and was crossing the line messaging in the evenings etc. I was going through a bad time mental health wise when this started and it made everything seem ten times worse. My anxiety shot through the roof and subsequently I had to access counselling (which I'm still attending) to work through it all. He asked this girl to stop messaging him and tagging him into things on Facebook and as far as I know it has all stopped, but it did take a few times of telling her until it stopped. I will admit I check his phone from time to time and there is no evidence of her contacting him via any other forms of communication apps (Snapchat/Insta/Whatsapp). Through the counselling I've managed to work through everything and we are trying to rebuild our relationship as it really did rock it. I have mutual friends with this girl and one of them has told me she has form for 'innocently' wrecking relationships. I have her number now as I took it off my OH's phone one evening when I was checking on him, so if anything else springs up, she will be hearing from me directly.
I just want to say that my OH is a very straight down the line person and I do trust him again as it is clear that he has adhered to what I asked of him and kept the friendship within work time only. Some people may think I was silly to go batshit over this, but there were lots of red flags on her part (things she had said to my OH when he was innocently relaying them!).
You often have to go on gut feeling and I know that if my OH had admitted to me that he had kissed her at all, then that would have been it for me. That's a physical affair, not just emotional.
If my OH had crossed that line as yours did and then appeared totally remorseful, wanting to rebuild the relationship, then I would be asking him to cut total contact with this woman. That means lifts to/from work, external contact outside of work and moving departments if he has to.
If he wants to salvage your relationship he should be doing everything he can to earn your trust by demonstrating he is actively cutting ties with her. You say you are going to Relate, is this the both of you as a couple? If you aren't going as a couple, then insist! If he doesn't, then I would resort to some of the other suggestions and contact either her or her husband directly.