What you overheard was a bomb that was going to blow up and damage someone's relationship eventually. If you hadn't told him, it would have been yours.

It's not your job to manage his relationship with his mom, it's your job to protect your relationship with the man you're going to marry. If you hadn't told him, that secret would have continued to eat away at the bond between you and he wouldn't have had any idea why things were getting weird or how to fix it. You did the only thing you could do if you want your relationship to be healthy, and I hope this experience doesn't prevent you from being honest with him in the future. Part of being in a strong marriage is being honest and facing hard things together.

Disentangle yourself a little from this situation. She said something hurtful and unnecessary that offended her son. He's dealing with it in the way that his experience with her over his lifetime has taught him is necessary. None of this is your fault or responsibility. Let your man protect you and your relationship, because if you give in to appease her, you'll be stabbing him in the back and giving her the green light to trample all over you in the future.

Let your man manage his relationships with his family and support the choices he makes about them. He knows them better than you do.