Welcome to the forum, tony andrews.
It's tough for parents of teenagers - most rebel about at least one or two things, and for teenagers with diabetes, blood testing can be one of those things. Is she taking her insulin doses? If so, while blood glucose testing is important, it's not as vital as insulin injections. What relationship does she have with you (and her mother) about other issues, as well as diabetes? What relationship does she have with other adults, including clinic staff, teachers, youth leaders, relatives etc?
The general pattern seems to be that most teenagers go off the rails for a while, then most return to manage their diabetes better, often after leaving home for study or work.So, for some families, the best strategy is to wait for situation to change.