will someone please help me understand why my friends acting like this and what i can do?

for the last 6 months or so one of my best friends has been like drinking more excessively and getting insanely hammered all the time, and when shes drunk she'll beg for more, like she will do anything for it, she says she doesnt want to lose the feeling to quickly (i presume by sobering up) shes always irritable and agitated and used to be way more calm than she is now like now she'll snap at the slightest thing and she'll get really angry and do this thing where she like puts her hand over her mouth and shuts her eyes like shes trying not to cry even if its like just stress at work or something shes 17 and she's starting sleeping around like not even desperatly just like she doesnt even care and then she says stuff like live a little but if any guy like tries to get closer to her she'll just shut them off like nicely but if they dont get the message she'll just tell them straight to back off, and she'll get into fights so much like even with security and stuff she'll kick off and she'll fight with people until the bitter end like she'd rather get her head kicked in than give up and i know she has problems at home but her mum seems nice and shes got a bit of a angry dad but he doesnt live with them but shes always arguing with her mum and walking out and stuff and then having no where to go and she used to be so nice and happy and just take the piss out of any shitty situation and now shes borderline nasty and she doesnt care about anything like seriously, and a couple of weeks ago she got horrendously drunk and one of my friends found her sat by a tree like bawling her eyes out and he said like has someone hurt you and whats happened and the usual but she couldnt even speak and didnt say anything, i dont know whats caused it like she broke up with this guy a month or two ago because she found out he'd only been seeing her for ages to make his ex jelous and then ditched her to get back with his ex and they didnt speak for a bit then on her birthday he text her and that led into like the whole 'im still your mate i still care about you if you ever need me just call me' conversation but that was when he'd fallen out with his now gf and then when they made up he didnt want to know and whenever shes drunk she ALWAYS calls him and his gf goes mad and hes like just leave us alone and she's got into fights with his new gf more times than i can remember and shes even tried deleting his number but when shes drunk shell do things like text someone saying its urgent she needs his number asap seriously and the other day she was near his house and my mate had to restrain her from going to 'just say hi and sort this out', also i know she lost one of her best friends back in december and idk if thats still a problem but thats when all this started and shes just going seriously off the rails and i dont know what to do or if shes just being a 'teenager' and i dont even know but someone who i work with whose also a nurse said she looks physically unwell i dont know what that means okay we're not even friends at the moment because i cant stand her drinking and her behavior and we had this massive argument and i told her that she disgusted me which i probably shouldnt have but she just shrugged and walked off whereas normally shed say something like yeh im a disgusting person an' whatttttt and i havent even got the time of day for her anymore but like what are your thoughts? sorry its long x