I wouldn't have a problem with telling his mom, once the papers have been filed and he has been served. Why should YOU have to suffer any ill thoughts from your MIL, when the truth is he cheated?
I would keep any discussion simple, private, and to the point - something like "MIL, you know I value our relationship and I have tried to make my marriage to your son work. But he is having an affair and shows no signs of stopping, so I have had to file for divorce".
Any questions she has, just tell her "I think you need to speak to your son about that"
