Might help if we knew the key specs on his present MBP, how it is "showing its age" and what his heaviest needs are.
It could also be, if it just feels a little sluggish, that a clean install would help. When I got the Mini, one obvious difference was how webpages I regularly access seem to be much quicker to refresh with new stuff than when on my Mac. Could have something to do with the flash memory. Could be the more powerful cpu. But I have a feeling much of it came from being a clean install. Or I could be wrong!
I also thing the biggest concern those looking to update have is, if they wait, does the change they are looking for (in this case, new Apple chips) happen when they hope it will.
Apple may well be planning on 2020-21, but that doesn't mean they'll reach that goal. And if they do, I've seen different theories about how they'll go about it. Some say they'll start at the low end. But some others expect it to start at the high end.
I do see some definitely logic for starting high. For one thing, if unexpected problems show up, it would more likely only affect computers they sell fewer of before they extend to the other levels.
Also, if these chips really are better than Intel, then if they start at the low level, would they be making them as good or better than some higher levels?
Anyway, all this means a person who feels they can wait 2 years (or whatever), may find they wait that long and it will be another year or two. At least that's the kind of stuff that always worried me.
For my first MB, I was waiting for Apple to come out with the aluminum 2008 models. Had it all figured out. Buy the mid-level MB. But when they came out, the reduced the cpu speed of the base model from 2.4 to 2.0. Yes, it was a better chip, but I had been expecting a better chip at the same speed. And the price to go to the mid level was $300 more rather than $200 at a time that I was saving every penny I could for my mortgage. So I stuck with the base model. Sure enough, later they went back up to 2.4 ghz!
Timing can be hard!
OTOH, I think times have changed. For a long time I felt a new computer every 3 years was appropriate - though I usually stretched it to 4 or 5. But, other than special stressful tasks, I wonder if that is still the case. Some games, maybe. But only if you really want a tiny minority of specific games. Movie editing, though unless you are doing it all the time, does a bit slower really matter?
It's like when I used to rip my dvd movies with Handbrake on that first MB. Would take 6-8 hours! So basically I could do one overnight and one during the day. When I got the iMac 5 years later, it was much quicker. I forget the time now, but at least twice as fast on that task.
But, surprise, I've finished all the dvds and won't be buying more so my biggest task isn't an issue anymore!
Actually, it's interesting that when I switched to the Mini, I did want to use an app (Tuneskit, I think) to make unprotected backups of movies I bought from Apple - but it won't work with the latest OS. So I did all that work on my old 2008 MB! Since no disk was involved, it wasn't as slow as doing dvds, but it sure would have been faster on my iMac or Mini. But it was a one time deal. At least until I buy another movie. But those are rare so just doing one isn't going to be a big deal.
I do like the general peppy-ness of my Mini, not to mention the 43 inch 4k display! Two desktops where, normally every window on each desktop is fully exposed. No more clicking to bring a need window to the front! Used to go through all kinds of organizational effort so I could always see at least a corner of each window! No more.
Now I just need for 2 TB external ssd drives to drop in price (Thunderbolt or, at least, usb-c). I'd prefer 4 TB.