as u/ThatGuyFromSweden said, the Clears are known to have this issue. I would try to send him a screenshot of a reliable source confirming exactly that. some elderly people love listening very loudly. My gf's dad basically blows his earsdrums out to mask his tinnitus when using his HD800. It's absolutely nuts. When I gave him my Hexa to try IEMs he literally maxed out my phones volume. I listen at half volume.

If that person continues to be super annoying I would say go there and simply take them back to spare yourself from the annoyance and sell them to someone else. Clears are popular, I think it's not too difficult to get rid of them and it would make yours and the dude's life much easier I guess.