DD is 15. She is a good kid. Well behaved, strongly disposed to help others, perhaps to the point of wanting to "rescue" them sometimes. Quite innocent and possibly a bit easily shocked. She doesn't drink at all, if it's relevant.
She is extremely worried about her friend, who is 16, and his use of alcohol. We don't actually know how much he drinks or how often, but he appears to do it alone at home and when he is stressed, to the point of making himself throw up. His mother is apparently aware of this, but does not seem to voice concern - though I can't be sure of this. I don't know much about the wider family situation at all, and neither does dd, as the boy tends to be quite private, but we do know that it's quite complex, not particularly stable and that relationships are strained. We also know that the mum has a new partner who the boy strongly dislikes - no idea of the reasons for this, however. As far as we know, there are two younger siblings at home, not sure about ages.
I have met the boy once - nice kid. Never met anyone else in the family. My only knowledge is based on what dd tells me.
I know that lots of teenagers drink alcohol, and don't want to overreact, but how concerning is this? I find it hard to judge as I grew up around a slightly "rougher" crowd than dd, and getting drunk at that age wasn't really anything out of the ordinary. However, it was mostly something that people did with their mates. Does the fact that this boy is drinking at home alone, rather than out with his friends, make this more of a safeguarding concern? Would you mention concerns to the school? Or just leave it?