I agree that it would be odd for this to come completely from nowhere, but if your relationship's otherwise fine, it's perfectly possible that she actually just had some very minor, standard DIL/MIL gripe on her mind, and that it got massively elevated into a huge deal in her head while she was absolutely hammered.
A bit like when someone is extremely drunk at a wedding, spots Cousin Becky laughing and putting her hand on their husband's arm on the way to the bar, remembers that Becky once flirted with their boyfriend when she was 12, and decides that Becky definitely wants to steal their husband and is a cheating bitch who will shag anything that moves and if she doesn't like hearing it they can take it outside for a fight and actually she's a fucking shit cousin anyway etc.
An 'Oh god, I'm so embarrassed, I have no recollection of that and I don't know what an earth I was thinking, I'm never drinking again' apology would be nice, yes. But I suspect it's actually the embarrassment that's stopping her from apologising - some people are like that and just want to never speak of the horror again. I personally think it's a coward's way out, but I can see the feeling behind it.
I wouldn't expect your son to 'make' her apologise or apologise on her behalf. It's not up to him to deal with his wife's mistakes or to order her to do/say something. It's between you and her.
I'm not saying her behaviour is in any way acceptable - it's not. It's bloody horrible to get a message like that, especially when you don't know what's prompted it, and she should be absolutely mortified and contrite. But I'm just saying there's a possibility that it wasn't so much a case of in vino veritas but more a case of in vino a tiny grain of veritas that was caused by said vino to grow into a giant monster that ended up not being veritas at all.