Family would be the first folks I'd try to contact, if that's at all possible. If she has one member who she seems particularly close to, I'd try for that one. I wonder if you asked whether she'd give you a name and contact phone number for a family member, you know, "Gosh, Mabel, if anything was to happen to you, I'd like to be able to get in touch with a family member, since they don't live close by. Would you feel comfortable giving me the name and phone number of whoever you'd like called if something was to happen?" Who knows, it might work, or you might learn of someone else you don't know about who is watching out for her.
If you can't identify anyone who she knows that you could call, perhaps your local Adult Protective Services could do a wellness check. They might have some leverage to get family involved if it seems needed. If her family is pissed that APS was called, well, you'd have called them if you'd known how to get hold of them, wouldn't you?