Oh dear, i am sorry.
I have to admit i dont know much about teenagers with diabetes, although i do have teenage children myself and of course once was a teenager with diabetes.
I never did this myself, if anything i would miss injections, not on purpose, just because i was so busy with what seamed like more important things at the time. Teenage years can be so difficult and hormonal, my son got very depressed over school and exams and several times threatened to take tablets or i would just find him crying. I suppose when you have access to insulin its just as easy to use this for self harm as it is anything else.
Unfortunately i dont have any answers for you. My son had counseling and did get over his depression once he left school and found his feet, he also felt able to talk to me so i could help him through, but not all teenagers are able to express themselves and tell you what the problem is. I am sure the hospital will offer you some help, if they dont then dont be afraid to ask for it.
I wish you all the best, please let us know how you both get on .