i think this is actually reasonable advice because in most systems, the application layer is pretty stateless and can easy be scaled out to more machines whereas the db is usually a central bottleneck.
I think too that this was his point, it only might make sense in very specific scenarios though, for day-to-day web-dev this is terrible advice as others have said.
I agree - on the majority of entreprise system (ERP) I have worked this was the case. I can confirm that GROUP BY are huge performance killers and sometimes the simplest solution is to avoid them by slightly changing the business logic if possible.
