My two cents... which won't get you anything...
Maybe a direct talk between M and J will help. If it's fear of rejection and judgment from M that's bothering J, and causing him to wall himself off in return, then maybe the sooner they can talk, and M can reassure him (if that's her intent), the better.
As for sharing in general, it really boils down to understanding what each person is comfortable (and UNcomfortable with), and working with that (or pushing back if you feel it's too restrictive). I know it's not a nice cut-and-dry answer, but it's all I got.
Well, that and those two cents.