Sounds like your girlfriend might have an autoimmune disease called Graves Disease. I'd have to know what blood tests she had in order to know whether she was tested adequately or not.. Graves Disease causes hyperthyroidism, whose primary symptom is weight loss.
Another possibility is diabetes, but it would depend on how long she's had the weight loss issue. If it's very long, she'd have other symptoms of diabetes.
A lot of people who actually have eating disorders insist that they don't and that they don't want to be that thin, but in reality, they really do want to be that thin. My daughter had an eating disorder, but for years, insisted that she didn't.
Of the three things I've mentioned, hyperthyroidism, diabetes or eating disorder (doesn't have to be anorexia) are any threat to anyone, but your girlfriend.
Does this mean I could contract Graves' disease or anything from her? Or is this all something that is non contagious?