Why doesn't he use Mate? Ubuntu Mate is readily available on the Ubuntu website. Is he aware that it's a continuation of the old Gnome (Gnome 2)? And well-maintained to boot.
Entertainingly, when I let him play with a bunch of different Linux desktop environments, the one he liked best was actually Haiku, which I threw in as a joke.
Really? I've installed Mate on ~10 computers, and have never run into UI bugs. I run Mate, my two kids run it on their computer, and I'm sometimes on that one helping them with school related stuff. Never run into UI bugs. I don't doubt that there are such bugs, but in my experience they're pretty rare (unless the project has taken a recent nosedive, always possible in open source).
I've run into some UI bugs using Mint Cinnamon, are you sure you were using Mate and not Cinnamon?
Do you remember what the bugs were? If it's something you can remember, I'll see if I can fix them so that future users don't run into them.
I'll certainly give Mate another try the next time I upgrade him.
Yes, I wouldn't be surprised if Mate is slower than Xfce, since it's traditionally considered more featureful. Besides, even using a cheap $200 netbook, desktop environment speed is just not an issue for me as long as I use linux + any reasonably fast DE.
When he had to switch from OpenOffice to LibreOffice he was pretty close to packing it all in and using something else; his argument was: why should he trust a package produced by an organisation that can't keep their own house in order?
That is, from the outside the schism between OpenOffice and LibreOffice is basically invisible; they're both considered to be the same thing, because they produce the same software. It's seen as an internal conflict, and it's a big reputational problem.