I am a senior in high school and so is my friend, well call her Eva (not her real name). I also live in America so the drinking age is 21. Starting sometime last year our whole friend group started to sometimes get together to drink to celebrate something or just to have fun, usually maybe once or twice a month if at all, and the only other time we drink is at parties so it's just like a group thing and none of us drink very often (maybe 4 times a month MAX). Some of our parents are willing to buy us alcohol because it's safer, and Evas mom lets her buy alcohol for all of us online with her card. Eva also lives alone because her mom is constantly away for her job.
All this being said, when we do drink together it always takes Eva the most shots to get drunk and when she's drunk she always craves more even when she's already blackout, she chugged like half a bottle of tequila one time while already completely wasted and somehow didn't throw up. During those times my friends and I would make jokes about her "alcoholic behavior" and she would get kind of mad and ignore them.
My main concern is that recently, starting maybe 3/4 weeks ago we would be texting in the group chat and she would mention that she was drinking alone at home. She only texted about it a few times but I wouldn't be surprised if it happened more times she didn't tell us about. Then 2 weeks ago my friends all went on a trip (I couldn't make it) where they all got drunk, and I guess since then she's not trying to hide it anymore because every single day since she's either told us or texted us that she's going to drink when she gets home.
She says it in a joking manner and usually says she'll only get "tipsy" and not drunk drunk but she doesn't buy anything under like 30% abv and at this point it takes her a decent amount of shots to feel anything at all and some nights she'll call and it's very clear she's past tipsy. My friends and I are all starting to worry a bit and the other day we went out and she brought a plastic water bottle filled with alcohol and drank like half of it even though she knew we'd be having dinner with my friends parents later. I've tried mentioning it casually or jokingly that she shouldn't drink every night, or alone, but she either gets legitimately mad/offended or brushes it off.
My concern with confronting her about it more seriously is the fear that she'll keep drinking but stop telling us about it which I feel would be worse, but I can't think of any other option so I don't know what to do. Any and all advice is greatly appreciated.