with all the hairdryers on at the same time, there's a lot of current being drawn from the mains to supply the dryers. This has overloaded a circuit breaker/fuse somehwere in his system causing a power cut or safety reasons. He probably needs to get a qualified electrician in and get the wiring sorted out so that it can handle the electric current at peak demand, or schedule the demand so that the overload doesn't happen. If he's busy, then I'd go or the electrician option to keep things moving safely.
