Education education education. Your daughter needs to understand about addiction, what it feels like, what it does to the thought process, and how powerful even the most seemingly inoffensive substances become once addiction sets in. She needs to understand that while it will be going on around as she gets older, it is actually illegal for her to drink now, and there are very good reasons for that. One of which is that studies have shown the earlier drinking starts, the more likely it is to end with addiction. Another is the way that some males will use it to lower a females inhibitions.
At this age kids most often get into trouble with booze if being drunk is the only time they feel happy or relaxed or just like they are having fun.
Talk about types of fun, and which are appropriate at her age. Find out whether she has enough access to fun activities, or if she and her friend were defaulting to booze out of sheer boredom.
Talk about the risk of allowing booze to become a default for relaxation, and other methods to help her wind down from the stress of school, social life and other responsibilities, all of which are increasing enormously at this age.
Ask about whether she is happy in general. If not, why not, and how can you help? Ask yourself how often you hear her laugh, or see her smile? Are you making sure it is often, and that happiness is her standard mode?
Ask about the friend. Does this friend drink often, or was this a one-shot deal? Check for lipstick on the bottle openings, you may be able to tell who drank what, or at least whether two different people drank one bottle each.
I definitely agree with telling her Mom, and discussing this as a family; but I get that this is not always possible. Do it if you can. It seems to me that the hardest decision you need to make is whether to tell the friend’s parents.
Bottom line, make sure she knows that she can come to you with concerns. Don’t let this conversation become angry or scary. Maybe the friend’s parents are abusive, in which case telling them would be a huge betrayal. Or maybe the friend needs help, in which case you can go to the school counselor or your pediatrician if you can’t go to her parents.
Or maybe they just tried drinking for the first time, and only drank one bottle each, in which case she just needs to know where this path can lead, and that you are a safe source of information and guidance.
I would also be very careful about phrasing how the bottles were found. The perception of a step-parent “snooping” in a teenagers room is freighted with potential negatives.
Good luck, and good-on-you for taking this seriously!