I disagree, I talk to my parents about my life, including if I am upset about something in my marriage, we are very close, they are supportive, it helps me.
I don't think this makes me weak in any way, I just have a good relationship with my parents. If my husband was thinking of divorce because of this I think I would tell him to go, as he cannot control whom I speak to.
Unfortunately, my MIL died some years ago, otherwise my DH would be talking to her a lot, in fact, I'd be talking to her, too, because she was a lovely, wise, supportive woman whom we miss dearly.
I think maybe a compromise? tell him which topics are entirely off limits but accept that he confides in his mother?
I guess it also depends what she does with the infomation, if she uses against you, then that's entirely different.