@Daughterpanic Sorry if this has been suggested, but is it an option to speak to this girl's parents?
My DD was obsessed with a boy she met, when she was 3 months away from leaving school (and she was still 15 - her birthday is mid July.) He was 18, and 2.5 years older than her, and going to university in the September.
They were 'dating' though not having sex (AFAIK.) However, DD, who was meant to be going to college in the September, announced in the June (just before she left school,) that she was NOT going to college, and was, in fact, going to go to her boyfriend's university town, and was going to live in his student digs with him, and was going to get a job in a shop and be with the 'love of her life...' 
I told her no, but she said 'try and stop me.' It was a really difficult time. Soooo, I phoned his mother, and spoke to her, and she had no idea my DD was planning on going to stay with her son in his uni digs. She was stunned. She also thought she was 17 and at college, and not 15, and still at SCHOOL. She was knocked sideways actually.
2 days later, DD's boyfriend took her out for a meal at Nandos, and she came back in floods of tears. He had ended it. She never knew why. He just said 'it's not gonna work with me at uni,' but she didn't know why, because he had previously agreed with her going with him! He said he had decided against it as he didn't want to be responsible for her not going to college, and affecting her education.
I rang his mother again the next day (when DD was out.) Turns out she had told him to end it, and especially as my DD was only 15! She told him also, that this was no way to conduct himself; dragging a 15 y.o. girl down to his uni/uni digs 100 miles south, and have her to work in a local shop whilst he went to uni.
Turns out he wasn't too keen either, and found DD a bit clingy, but didn't have the heart to end it. He told DD it wasn't going to work with him at uni, and it's over.
I think she would have gone batshit if she had known I had spoken to his mother, and that resulted in him ending it. To be fair, it really really needed to end!
DD went to college, and uni, and when she was at uni, she met a lovely young man who's the same age, and only lives 20 miles away from where we lived. She is still with him too. 5 years later. 
But she never knew I spoke to this other young man's mother....... And she never will.
So yeah OP, see if you can talk to this girl's parents...