I don't know what I'd do. I would ignore it for now - the text is just lashing out, if the wife only learnt about it today - plus Christmas is a bit of a weird, emotionally-charged time anyway.
Were I the OP, my loyalties would first be with my wife. I would expect her to remain loyal to the OW. I would want to stay loyal to the H, as he's an old friend, but I don't know how I'don't feel after this, whether I'd still want to. But it is the W who deserves most support. And if she takes him back (which she has currently, but may not stick to it when she's had time to reflect and learn more,) and she insists on her or OW, then it's going to break either the fronds hip between OP and H or the friendship between DP and OW - or it could put OP's own relationship at risk. Or a combination of all those.
There was never any way that an affair between two people in this group wouldn't cause massive upheaval when it came out - and eventually, they always do. I'd be livid with the H and the OW for being so stupid. An affair is never good, but the repercussions and feelings of betrayal in a close group like this will be much more magnified than if he had gone off and had an affair with a stranger - and you don't need much emotional intelligence to know this, so they knew what a risk it was.
If I were the OP, I'd be thinking about when I found out, compared with W, and if I knew before, am I complicit because I didn't tell her? What about DP? Did she know? When did she tell OP? I'm guessing she wouldn't tell W anyway. How will it affect OP's and DP's relationship? Is there any issue with knowledge which wasn't shared as soon as it could have been? What if DP insists on sticking with OW? Will OP be okay with losing H's friendship?
I'd hope that by not reacting, and Christmas and stuff, things will blow over a bit and at least the friendship ultimatum will be dropped. But it might not be. I don't know which way I'd go in that case, because I think it would depend just how much I would grieve for which relationships, and what conflict it might cause with DP if she wants to decide differently, because some preferences about who to side with won't be compatible with others. It might be I'd feel sufficiently angry with H that it wouldn't be so difficult to stick with OW. Or vice versa. But currently I'm not in any friendship group where I am so close to all the parties that I would feel so torn - all the couples and groups I know, I know one person more than the others, and I can't imagine not going with the friend. Although I did cut out an old friend because of how she treated her husband when they split. I'm not very tolerant of lying, and this was to a point I could not forgive.
I don't envy OP at all. Good luck with it all.