The dude said he only had a 45. So based on that it looks like you agree, that he should keep a solo pygo.
@OP, there's absolutely nothing wrong with keeping a solo pygo. You will notice that they are more interactive, bold and aggressive when kept solo.
For those who believe that they "thrive" in groups in the home aquarium, because that's how they appear in the wild, it seems like a common misconception.
Quoted directly from OPEFE:According to world expert on piranhas, Dr. William L. Fink, piranhas do not shoal in the literal sense. Piranhas form groups or more specific a school of individuals having commonality. See further remarks my own opinion of pet stores and how they describe what they see.
Pygocentrus species form small groups of individuals that stay far apart from each other and group together when feeding frenzy or during fish runs. The same goes for Serrasalmus species. The aquarium is an area that is not the same in nature. You, the hobbyist, can jam the species and call it a shoaling but that is not what it really is, more like forced grouping. During the fish runs (in the wild) schools of fishes, including piranhas are seen. But again, this is nature putting them together, where in normal circumstances they are kept apart by natural processes and behavior.
So I cringe each time I read that someone is shoaling their fish (pet dealers are notorious for this assumption).
Good luck with your decision.