hi there,
On balance, I think I wouldn't say anything to her, does she know her son realises she gets drunk? if she drinks around your son at all then on some level she does know this, it isn't new information, and it would be reasonable to surmise that he is going to talk to friends about it because that is what people do, especially kids.
by telling her would you be trying to attempt to change something? shame her into realising what she is doing is affecting your son? on some level she already knows this. I know this from my own experience: I used to detail what stbxAH did the night before, at first in an angry tone, later just imparting the information. I told him that he had passed out whilst looking after our son whilst I had gone to the gym for 20 mins on a saturday morning and I had come home to find our 3 year old standing over him screaming - he felt terrible shame over this, terrible remorse, but it changed nothing.
later I realised that he used any information of this kind as an excuse to drink more to drown his remorse: if the activities of the previous night were not too bad but still shaming, he would ring around friends and family to confess them, and his remorse and get emotional strokes to bolster himself, usually whilst still drinking.
so check your motives. I think if you feel there is a possibility she will use this against your son, then that would be the clincher for me not to say anything.
talking to your son about how he is handling this situation, suggesting alateen or therapy might be good thing though? what do you think?
Good for you on your response to your son by the way - that sounds spot on