I can't tell you how to feel but I would be very much discussing it with him rather than accusing him of anything or leaving it by considering it a father daughter dynamic. Whatever anyone says the father daughter, mother son dynamic is often only in the minds of one, not both, in these relationships. I know plenty of people who would keep their occasional fantasies to fantasy while the boundaries are kept to but any blurring of the boundaries like discussing personal issues that suggests she might be available or just needs a hug can send them off in a tail spin, a broken marriage and a situation that had their blood not left their brains to service other organs in that moment, they wouldn't have wanted the situation they end up finding themselves in.
If they are both stressed they are not helping each other only winding each other up. He and she needs time away from work and worries about colleagues to regenerate and be on the ball for their job. If he is stressed could it be because she is stressing him out with her woes or vice versa. She needs help from people who can do something about it, your DH can't do anything and he is not her confessor either. She and he are building a bond here and that would make me consider taking action sooner rather than later.
I'd be trying to show an interest in why he is so concerned about her, how you can both help her, not just him and by this I mean professional advice on her worries not both of you shouldering her problems. That bond needs to be broken between them so they can get back to being professionals and colleagues not dependant on each other for emotional succour. Don't accuse them of anything, try and put a positive spin on it that you are interested in what is happening to them both and how you can help DH set strong boundaries so it doesn't tip into emotional territory
I'd also consider if something is lacking in your relationship that he is looking for elsewhere. There is usually an intimacy or communication worry that causes people to look for someone more amenable, it doesn't necessarily mean they intend to have an affair but it can be how it starts. How can you both work on your relationship to strengthen it? If he is sharing information about your relationship with her, then you need to act now as it is a sign that this is not platonic on his side and he may not yet realise that.
Emotional affairs can affect not just the marriage but the workplace too. Teams and colleagues aren't stupid, they will know something is not right and will act accordingly, they might feel he makes her a priority or they get fed up with hearing all the whining, someone somewhere may complain to HR and then there is a career issue for DH or his work wife in career progression or even gross misconduct. No one thinks about that when they are creating work bonds, I've seen many lose years of their career over a silly crush or a complaint from someone who is hurt.
You and he need to create a strong boundary about this and ensure he doesn't cross it again. That starts with conversations outside of work hours and conversations about you and your life together ending right now. Use the words "I" rather than "you" when you want to to prevent sounding accusatory, tell him how worried you are for him and offer your support. Ask him if he has any concerns about your relationship, don't mention the work wife on that one, ask that seperately. It is too easy to say things like "you always put them first so stop it" rather than "I want to be there to help you because I am worried about you and I love you". This helps to keep commnications open so it doesn't drive partners into the arms of a colleague.