Why would someone do it? I think there are people who would prefer their child drink at home, rather than outside somewhere, figuring, "They're going to do it anyway". Now don't get me wrong, I don't agree with it, but I have heard it said before. I think they're foolish. Maybe they need "education" about Social Host laws and their implications (if your state has them). As for the friendship? Well, I don't think I'd care to be friends with someone so misguided. I could probably maintain a professional relationship, but I'd tell her what I think of her giving beer to my son, first. Not advising you to do this, necessarily, but the police might be interested in their "Contributing to the delinquincy of minors", including your son.
If someone is hurt or killed, you might wish you said something.