Unfortunately what you are asking is a very legal issue.
Is PB a "bad thing"? Ask a plumber and he says yes. Ask a lawyer, he says if blah blah and before blah blah and pipe marked blah blah but if you had blue pipe with green fittings blah blah.
It seems that home inspector reports spend much time with legal disclaimers that they can't see this and they couldn't move that and they are not responsible for anything. I suppose they occasionally mention a bad condition that the buyer might not otherwise have been aware of, but the horror stories seem to outnumber the success stories. Of course, we only hear about the problems.
In this case, I guess my main comment is that I would think the inspector could have recommended that the purchaser do his own research on the subject, or call in a licensed plumber for further evaluation.