What a terrible shock it must have been, I'm so sorry. I'm glad to hear he's in a safe place, time and medication can do the rest.
Diet pills are stimulants, like amphetamine or "speed", and they can cause a similar psychotic state called Stimulant Psychosis
. The big offender in alot of these pills is something called ephedra or it's Chinese name, Ma Huang. It's been banned in the US and the UK but it's still being sold and used quite openly. Hydroxycut has had so many lawsuits over side effects I'm stunned they're still in business.
Here's a Pub Med report from Ireland on a man taking ephedra based diet pills who experienced psychosis, more items to the right. http://www.ncbi.nlm.nih.gov/pubmed/11693229