@boomerang,
Goodness...I don't know that you can do more than you already have.
Sounds to me as though you have covered the bases.
I think separation/divorce is just a pretty overwhelming experience for most kids and Mo is, as you say, going to be a kid for whom such happenings are very triggering. I think you may just have to ride it out with him.
I'm sure that experiencing his friend's grief is very hard for him, but what are you going to do?
Have you discussed the grieving process with him, so he knows his friend will gradually start to feel better?
I use a train going through a tunnel thing with kids.....you know, at first you can see only darkness ahead and the only light is from behind (the past and how things used to be), then being in the middle, where it feels as though there is no light, because you know things can't go back the way they were but you cant imagine a way ahead, then gradually you see faint light ahead and eventually come back out into the light. That sadness for what you had doesn't completely disappear, but it gets softer and easier to deal with and you gradually don't think about it much and can see good things about how things are now.
Thing is research is suggesting that for many kids the pain remains pretty sharp for a long, long time.
Is he talking to you guys about how he feels and what it is like for him supporting his friend?