Eyebrow thinning is not likely related to her diabetes even if she were in poor control. I would suggest discussing this with her primary care physician and perhaps seeing a dermatologist. In terms of mineral and vitamin deficiencies, these can be associated with eyebrow and hair problems. Getting a good sense of her vegetable and fruit intake gives a good sense of how much of these nutrients she is getting on a regular basis (or not). A general laboratory screen, blood count, sed rate, CRP level all might be worthwhile to check for any subtle underlying problems. Likewise, the same with thyroid functions and celiac antibodies, if not already checked. Blood vitamin D, zinc, and magnesium levels could also be obtained based on that specific history and, if these levels were low, then I would specifically think of supplementing the ones that are too low. A good multiple vitamin/mineral such as Centrum A-zinc or similar would also be reasonable but it may take 3 to 6 months to see significant change in hair/eyebrows. Many of our patients have significant vitamin D deficiency and when they are compliant with taking daily supplement and the blood levels move from very low to target range, then we often see hair and skin improvement, sleep improvement, less anxiety and depression, less ADD/ADHD as well as improvement in lipids and blood pressure readings.
Additional comments from Brenda Hitchcock:
Given her age, I would also try to find out if she has been plucking her eyebrows. It could be that her peers have already started to do so, so she tried it to fit in.