Unless your friend is really an avid seaman who is going to actually go places - I'd probably also recommend against this kind of life. My sister and her husband are a lot happier (and more ACTIVE) now that they are back on land.
If I had to do it, I think I’d just get a Eurail pass and travel around Europe on trains and occasionally stay in an AirBnB for a few days.
Not an avid seaman, but smart and good at doing just about anything he sets his mind to, from my observation. A driving factor is that they aren't going to be able to maintain their current large home and grounds. So what to do... That's what has him thinking of alternative options. My suggestion to him is an RV, but his thinking is if SHTF, an RV is going to get you stuck somewhere you might not want to be, as roads may be impassable.
So, he has a lot he is considering. I have an RV so I'm perhaps biased IMO. I've seen some mighty good ideas, mentions of things to consider on the thread and it's going to be helpful to them, I'm sure. Everyone's situation is different of course, and we each have our dreams and aspirations in life.
One thing for certain, have a backup plan if things don't go as planned.