So many of the signs the OP suggests do, of course, align with diabetes, however there are other conditions where a presentation of "not looking well", losing weight, dark shadows under her eyes, apparent extreme thirst, an apparent increase in weeing and an apparent inability to participate in her workplace to her usual standards.
From the top of my head there are a few things I could match to that description, with everything from burning the candle at both end, to drug abuse to an eating disorder. Nobody knows, although the young woman herself might be able to narrow the scope a bit.
Whatever the issue, she needs help, but whilst she has capacity. nobody can make her seek or take it. Hard though it may be to witness, if she will not consent to going to A&E, her Doc, a counsellor, or whomever, there is likely to be very little that can be done about it.
In my view there is a fine line between helping her and driving her away because she feels harassed. The sad fact is that if she is effectively sick or self-harming, if she doesn't consent to intervention she has the right to continue down what could be a route to a potential disaster.
It's not an easy one, and almost certainly not something that can be addressed or solved on an online forum. It may be a case of waiting until she is in an acute crisis and her will is broken.