As far as treating a jellyfish sting our suggestion is the “beach method” which involves no chemicals. Take a hard surface something like a credit card/room key and scrape against the effected area. The goal of this is to remove any remaining tentacles. Next take wet sand and pack it on top of the affected area and let it sit. This relieves stinging and adds a cooling effect.
Realistically a jellyfish sting is very similar to a bee sting. Pain thresholds are different person to person and pain tends to last for 10-15 minutes. In the case of an emergency: person has difficulty breathing or breaks out in extreme hives all over the body call 911.