This kind of project is typically called a "wellness check." You'll find quite a few similar projects in the forums. At one point there was a community member who worked with the VA who came up with a number of really interesting ways to monitor whether someone in a house Was going through their daily routine.
One of the absolute simplest is just put a sensor on the refrigerator. Some other options are pressure mats in the bedroom to make sure someone has gotten out of bed, a contact sensor on a drawer where daily medicine is kept to make sure the drawer has been opened, all kinds of things. I'll put links to some of those projects in a minute.
The baseline question: what reliability do you expect?
All of that said, as a medically fragile person myself at the present time I would not consider SmartThings reliable enough to function as a wellness check for someone who really needed one.
Since last October I have yet to go 10 days without an impactful SmartThings failure. Sometimes all that is required is taking the batteries out of a sensor and putting them back in to reset it – – but since I have very limited hand function, I have to pay someone else to do that. This makes me very aware of these issues.
SmartThings also does occasional false reporting, and that could be very dangerous in a wellness check system.
Also, even when everything is working perfectly, SmartThings can and does push out updates to the home system which can take it off-line for anywhere from 15 minutes to several hours. You cannot postpone or refuse these updates. Sometimes we get notification a few days in advance, but not always. This kind of interruption is often unacceptable for a primary medical monitoring system, so it's something to be aware of.
When SmartThings works well, it is my favorite home automation system, but the reliability just isn't there yet for a medical support system. I now use mine for noncritical notifications like getting a notification if the guestroom window was left open when rain is expected.
So I just wanted to mention all of that. If you search the forums for "stability" or "reliability" you'll find many discussions of these issues. Still, you might find some of the following ideas helpful for secondary monitoring.
OK, onto the good news. Here are links to some of the other projects people have used.
I'm sure there are more of these, but that should be enough to get you started.