To answer this question with any kind of accuracy, I'd really have to know the person in question.
I learned bass (and guitar later) entirely by myself, first by noodling around, and eventually collecting stray bits of information that came my way until I had a working grasp of theory, which grew from there. There are wholes in my knowledge, but they are primarily issues of semantics. For example, I understand and know all my modes even if I forget their names, and I know a lot of concepts in practice that I can't necessarily properly explain in terms of hard theory. This is all stuff I'm working on. Every year I feel like my grasp of music solidifies substantially, and I'm proud of the fact that I did it all without a teacher and without a class.
So yes, I think it's entirely possible to get good without instruction, but you have to be motivated, and to an extent you have to be lucky. In my case, I was lucky to be surrounded by musicians who regularly talked about the science. I just listened quietly and absorbed as much as I could so I could take it home and mess around with it. I was lucky enough to care to learn what was happening with music, and lucky enough to get the information I got in a correct order and at a correct rate that allowed me to actually use it. Eventually I learned more still by playing along with other musicians and by trying and failing at a handful of things.
And you can say that I would have been better off with a teacher, but I'm not sure that's the case. Even though I think theory is a valuable tool, and proper technique is considered such for a good reason, I don't think I could have learned it unless by trying and failing, and doing things at my rate when I felt like doing it. It's just my personality. I'm too stubborn to let anyone tell me what my assumptions should be, regardless of their credentials. I need to try every possible bad idea first, and know for a fact that they don't work. And I actually think that the actual quality of my learning has some advantages because of it.
HOWEVER...
I have been blue in the face trying to get some self taught musicians I know to take their work seriously. Some people don't ever find motivation to learn the ins and outs of it, and if that's how it is, that's just how it is. And while he may learn some concepts informally that help him similarly to theory, he might never progress even to the level of a 3 month piano lesson veteran. There's no way to tell without actually knowing the guy.
One thing's for sure, though. If he doesn't find the motivation by himself to learn (either by really driving himself or by taking lessons), he's probably not going to. And you can't really do much about that. But also, if he's having fun, there's no reason you should mind. Music is serious business for some of us, but some of us just like to mess around, and that's cool too.