I'm not sure how you still think it's overwatering if the mix is 50/50 perlite/peat and he's waiting until at least the top of the mix is dry before watering again? I'm not saying it's def. not overwatering, I'm just not sure how it could happen given the mix and his watering habits?

Unless he's leaving the pots sitting in water in the trays pictured after watering, or the pots have no way to drain.