Yes, as others have said, it's important to be legally granted the right to make emergency decisions in case of medical problems that may arise.
Something that your daughter and her husband might consider is a program called Emergency Assistance Plus (don't worry, I'm not selling it or linking to it - you can google it).
We all have it in our family. It is NOT health insurance, but it is travel aid in the event of a medical problem while the covered person or family is away from home. Our adult son has it because he often travels for work, and my husband has it for the same reason. I don't travel much but dd and I are covered through my husband's family plan anyway.
What it does is offer help in the event that the covered person is sick or injured away from home (it doesn't have to be foreign travel, just not in your regular commute; if you live in Dallas but had to travel to Seattle for a conference that was not routine, and got hurt there, the plan would help with that). Services include things like sending a rep to your hotel room to pack up your belongings and bring them to you or ship them home, and check you out of the hotel (in the event of a hospitalization), flying a family member to be by your side (in the event of a serious injury). They provide translation services (imagine being flown from your cruise ship to the nearest place with a hospital and trying to communicate your medical history in Turkish or Greek or Japanese - medical lingo can be difficult enough in English sometimes!). They'll arrange to return your rental car, or help you get back home. They'll help if you lose your passport. If persons are on a cruise, and one gets sick or hurt, they'll send someone to retrieve your belongings from your room. If you can't get home due to illness, they'll arrange for pet care. They'll help fly you to a better medical facility if you find yourself seriously ill or injured in a place where the care you need is not adequate. They'll help find lost luggage, and help replace lost prescription medications.
The only caveat is that, aside from 911 (or the emergency service in whatever country you're in) required for life-saving rescue, you must call and activate the Emergency Assistance Plus number prior to calling anyone else. That seems simple enough. And anyone with airline reward miles can get a discount on the Emergency Assistance plan. A family plan can be less than $200 a year. I think it's a pretty amazing program.
I just wanted to let you know about this service. As I said, I don't sell it or have any connection other than having peace of mind from having it in place.