Not sure about the circumstances with your MIL and how she and her husband split, but I know that in my own situation, my dad just told my mom that he didn't love her anymore and then peaced out. They had been married for 35 years, and the same month he moved out, he started living with someone else (who, as it turns out, had been in the picture for years). Just because my dad didn't die, doesn't mean the shock is any less for my mom. If he had, at least she would have thought she was still loved, instead of completely losing faith in herself for making the mistake of staying married to a cheater.
I don't see my mom dating in the next five years, but I wish she would. She is so devastated by this that she says she can't imagine ever trusting anyone else. So just because it isn't a death that breaks up a marriage doesn't mean that the person is ready to move on and date.
It sounds like it's just a sore spot with your MIL - maybe she was afraid she would say too much to you, since it sounds like you're the only one she confides in about this stuff. She might have just snapped at you so the conversation would end. Either way, I wouldn't worry too much about it.