Chances are if the young lady is diabetic the gangrene was caused, as previously mentioned, because of lack of circulation to the extremity (leading to lack of oxygenation to the extremity); however, there are cases of gangrene that have a bacterial origin, as with anaerobic bacteria that burrow deep within the bodies tissues.

http://www.merck.com/mmhe/sec17/ch190/ch190g.html

http://www.uhhospitals.org/rainbowch...7/Default.aspx

Even if she did have something that was contagious, if she was started on antibiotics, she will (in most cases) not be infectious after 24 hours (that's a general rule, not always applicable, but usually).

Hope that helps