if you ask a carpet fitter to replace the carpet, the first question they would ask is "do you want new underlay"? and then they'd speel as to why underlay needs replacing, personally if your doing a job for someone then its up to you to make sure all the facts are correct before commencing. so allthough i wasn't there and didn't hear the conversation, my answer to the post is your in the wrong.
i'm pretty sure that even if i hadn't initially asked if he wanted to keep the underlay, once i'd ripped up the carpet i would of then been reminded to ask and would of asked or phoned him, if i couldn't of got hold of him, i would of rolled underlay up and put it too one side till i could ask. i would also be in a position to let him know if it was worth keeping or not.
hope this helps