Not necessarily so ... and it's an assumption that doesn't take anything about that diver into consideration.
FWIW - I've known, and trained, some divers I would inherently trust as a dive buddy far more than some old-timers I've known. Being a good buddy has less to do with experience than it does attitude and habits.
Much the same can be said about the scenario posited in the initial post. You can't tell much, really, about a diver by looking at him. And you can't tell much about how safe he's likely to be by the gear he's using. Having a pony bottle is only worthwhile if you have the presence of mind (and the ability) to deploy it in a timely manner in an emergency. I once dived with a fellow who told me his solo strategy was to dive until his main tank runs OOA, then switch to the pony and come in. I'm sure he wasn't trained that way ... but even after I explained to him why that wasn't a very good idea he continued to not see anything wrong with it. Divers like that can't be reasoned with ... and it's best just to stay away from them and let Darwin sort it out.
The problem with people like that is that they aren't just a danger to themselves ... but to those around them. And you don't have to be someone's dive buddy to be accosted by them once they put themselves in a position to discover that they're not nearly as well-prepared for an emergency as they thought they were.
So no ... give me the new diver who paid attention to his training, and who is working hard to improve any time over the older hand who thinks he's got it all figured out and routinely cuts corners. Chances are pretty good that even in an emergency, the former will be better prepared to think their way through the emergency without creating an even bigger problem. A good attitude frequently trumps a good number of dives in that respect ...
... Bob (Grateful Diver)