Not to second-guess your mechanic, who has seen the failure in person, but over-inflation is probably not the first thing I'd think of to cause of a failure at a spoke hole like that. I suppose anything is possible, but a spoke hole is a pretty high-stress area. Spoke tension can contribute to fatigue over time, but yours is a pretty new wheel to have failed from "normal use". It looks to me like there may have been an issue during manufacture (perhaps the hole wasn't drilled clean and stress fractures started). Those rims should be able to handle well in excess of 100 psi.