Yes - this happened with both my grandparents. I'm sorry I can't help more as I was teenager at the time and wasn't very involved, but I do remember my mum talking about it.
My grandfather used to get very aggressive with my granny, but he had always been difficult towards her so I think she was used to putting up with it (not saying that made it easier BTW). Also he was very ill and didn't live for long.
Then my g'mother had dementia and came to live with my parents and they were a lot of incidents of her trying to beat up MY Dad. I think she also accosted a policeman once and high karate kicked a hotel owner when they took her away for a holiday once...
Could your MIL keep a diary to see if there is a pattern on the day? If they happen in the evening when he's tired? Is it when he gets particularly frustrated, maybe at himself?
It may be the Contented Dementia book that CMOT has mentioned but I heard someone on the radio the other talking about how usually we deny what the person is saying "No, you don't go to work any more, rememeber, you're retired" which just leads to more confusion. There was also a nurse in anursing home who siad they had eg, a woodwork bench for a man who used to do a lot of that and he would wander over, happily pick up and sand a piece of wood and then put it back again, until he did it again. And a lady who used to be a nurse and insisted on helping withthe medicine round who now has her own medicine trolley with cups of water, smarties in jars. It sounds a bit patronising but much less antagonistic.
Sorry - long post, but difficult subject. Hope you find some help.