Not an expert, but...

What does the manufacturer of the dialysis machine recommend? (Either specific brands/models, or generalities about the quality of power needed by their machines.)

In many areas, "power failure, and resident needs power for a home medical device" is a situation that the local Fire Dept. / EMS / Utility regularly handles, or has protocols for. I'd check with them. If nothing else, if things really went sideways and they were responding to your FIL's home, having a UPS that they were familiar with could be a real benefit.