My friends have a family member (who is now in his 50') who has this similar problem. He's completely self sufficient, has his own apartment, comes and goes at his discretion, is the most pleasant fellow on the planet, but cannot handle any money whatsoever, cannot handle his banking, cannot make change, (can't spell either), so he can't do his own money management whatsoever.
His family controls ALL money matters, pays his bills, etc. He is on SSI, and the other family member is the payee. The man with the money management problem, has no idea this is happening because he doesn't know that he himself has limitations. He thinks everyone has other family members that handle money matters.
So they give him a weekly allowance and they gave him a prepaid visa. He used he whole thing on nonsense items because he didn't know any better.
The family became furious, took it away and now he is depressed. He also has his own phone and ran up such long distance that everyone became furious.
If you think that your family member is able to use a prepaid visa, then perhaps you can put let's say for starters, $100 on it. And then see how she handles it.
If she only has $100 to spend and you explain "this is for emergencies", and she GETS it, then that's half the battle, right??.
You haven't done this yet, right?
Also, the idea of getting a debit card that is linked to her own bank account is a good idea IF you go with her to open the account speak to the person opening the account, tell him the whole story (in private of course), and see if there is a way that she can't do the overdraft thing, etc. etc. Perhaps they have special bank accounts for special people. If they don't perhaps THEY SHOULD HAVE.
If you find a bank that does this, than that's half the battle, because you can add funds from any place into her bank account when you travel. And your mind will be at peace.
I don' know how old she is, but perhaps this can be a money management learning experience for her, and when she does something GOOD, you can praise her and she'll feel good about herself (positive reinforcement I believe it's called).
Whatever you decide, you are a true straight shooter and a very nice person to be so concerned for her welfare.
I suspect all will be fine with you in her corner.
All the best,