My SO is going through divorce. I think the best thing I can do for him is be a sounding board as he works through it. There are no magical cures. :/ Even if something is logically understood, it takes the emotions a long time to catch up.
The other thing, and this may depend on temperament and circumstances, is to give him his space when he needs it, and not be overly demanding of attention. In my case, my guy has children and helping them successfully weather the transition is his focus. That is as it should be and I respect that.
I am sorry. I hope this helps. Unfortunately, there just isn't much you can do, but love him and be supportive.