I know some concerts and places that play/hose live music can be very loud. A friend of mine who has sensitive hearing (can hear a pin drop from the other room!) is going to a concert in Vegas in early January. I suggested maybe using earplugs but he is insisting he won't need them.
They are cheap and take up next to no space in your bag, so just tell him to take them. Even if he doesn't use them, he will be glad to have them if he need them.
^ agree
I think your friend needs to just get some in case. They are so cheap these days and take up no room. I think some hotels offer them as well for free so he can just get a pair before going to the concert.
I always keep some with me when I travel, just for the sake of staying in a public hotel! So yeah, just tell your friend to have them on him. They take up no room and aren't an inconvenience to carry around.