I can understand grieving that even a friendship isn't possible and being sad about how many issues she has, but hadn't he already decided that he wasn't interested in a romantic relationship with her, and that it was basically done out of convenience in the first place? I guess I don't understand where this strong depression is coming from. It's good that he talking to a therapist- if this has deeper roots than the loss of one relationship hopefully the therapist can help him figure it out.
I don't want to say to someone, "Okay, your time to grieve is up, snap out of it" because I know it doesn't work that way. But at the same time I feel like celebrating the positive conclusion of the 8 month custody battle should take precedence over grieving a relationship that ended. Particularly when it's already been 2 weeks, and it was not a "life-partner" type of relationship. He may not be able to *feel* as happy for you as either of you would wish, but as a grown adult, assuming no mental or physical health issues, he should at least be able to be there for YOU in your happiness as he wants you to be there for him in his grieving.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack