I know this won’t be especially helpful, but it is totally normal for her to be acting this way.
I’m 37 now and was diagnosed at 7, so not quite 30 years T1D.
When I went to college, I was convinced I could just “live like everyone else” and not really worry about my diabetes. I was on a pump, and I still filled it and took boluses and such, but I almost never did a blood test, and I certainly never said “oh wait, I should eat first” or anything similar.
My “crash” happened when I dropped so low on a weekend my roommates called my parents for help. I was so embarrassed by it I vowed not to ever have that happen again.
I honestly believe the message just can’t come from you. Nothing you say to her will get through because you have been the one helping her from the start. She’s “rebelling” just like we all have done in life. Everyone has a rebellious phase. Unfortunately, the way T1Ds tend to rebel is by pretending nothing is wrong.
So, you reach her the same way all rebellious phase people are reached: send a messenger. Maybe you are close with one of her friend’s parents and can plant the idea there? Her friends are the first ones who can reach her. Maybe a teacher or a coworker she respects? Someone whose opinion will matter. Not a doctor or a parent though. She’s probably rebelling against all of that, too.
Oh, and one small silver lining for you. Even though drinking at this age is self-destructive, it is probably also helping to keep her blood sugar lower. Liquor generally lowers blood sugar about 8-12 hours after you drink it.