A friend wore regular socks on the trampline at an indorr bouncy house, and did sign the waiver. However..

After searching the internet I found trampoline manufacturers strongly suggest as well as other bouncy houses REQUIRE nonslip socks. He severely broke his arm, requiring surgery, will miss work, and the pain is excruciating. All of this could have been prevented had the bouncy house required non slip socks. Does he have a case for negligence, though the waiver was signed? This is a vey common source of injuries and must be stopped, and could be with a $1.99 pair of grippie socks. Any advice is greatly appreciated.