Nasty situation.
Wasn't sure if I should say anything, as it reminds me of when my exh's mum died, which led to our divorce. But maybe that gives me an insight into what I could have done diifferently, so might be useful.
After my ex's mum died, he started talking to a woman at work about her death, so he said, for support. I couldn't challenge this, as you can't accuse someone who's grieving of having an affair with the person who's supposedly supporting them in that grief. But that is indeed what he was doing; as I found out later, they exchanged hundreds of emails, none about her death, and started sleeping together soon after they met.
Clearly, your dh might very well be totally innocent and not doing anything like my ex. But whether he is or not, my advice would be the same at this stage - to treat him as if what he is saying is definitely true, and to do your best to support him. Because if he was sleeping with her, you being suspicious, accusing him or stopping him from contacting her would not change that: he would continue secretly and feel justified in an affair because of your suspicion. And whether or not he's having an affair, he is still a human being suffering from trauma, and you are a decent person who knows him well and thus might be able to support him.
I'd support him as best possible, treat him as innocent, keep my eyes open, stay well informed about my options if something is up, and see what happened next.