Allyn, your husband may give the impression of being the 'Life & Soul of the Party' in the eyes of your friends. But the fact that a man of his stature can condone another [of similar height] - regardless of whether or not it was his dad - launching a series of physical assaults on ANY woman ... let alone one who is much smaller built - indeed his own wife ... and treat the whole thing as a is totally inexcusable. And to stand by and watch a repeat of such behaviour, amounts to nothing less than downright cowardice ... an issue that needs to be confronted at THIS stage, to (hopefully) prevent it occurring again!
Aromulus mentioned the "Glasgow Kiss". I happen to be a Glaswegian by birth but, having moved away from that city when I was only 8 years old, I cannot claim to be familiar with its reputation for being a tough place to live. In the present context, I'd guess he was referring to the practice otherwise known as "headbutting" :HeadButter: ... whereby, the circumstances here, merited the son doing "just that" to your father-in-law, as a natural reaction to the latter striking YOU.