I think there is probably a difference between a "private friendship" that meets often with just the two of them - and people who grab a coffee once in awhile.
One thing I would recommend from painful experience is patience. His first few weeks and even maybe months with a therapist are not going to be super helpful; it typically takes time to get through the surface churn before you get to the stuff that is really going to help him deal with the issues.
A second thing is to just be calm and cool and leave it alone, especially the Friday night confrontations. It doesn't sound like going dark is needed in your situation, but I would keep that stuff to a minimum. Right now, this OW is a source of emotional support, and it's unlikely he will want to walk away from it.
