Final answer:
The most appropriate immediate action to take when a friend is stung by a jellyfish is to apply fresh water to rinse off the stingers. Immobilizing the arm and keeping it lower than the heart, as well as calling 9-1-1 or the designated emergency number, may also be necessary depending on the severity of the reaction.
Step-by-step explanation:
The most appropriate immediate action to take when a friend is stung on the arm by a jellyfish is to apply fresh water to rinse off the stingers. This helps to remove any remaining tentacles or stingers from the skin and minimize the amount of venom being released into the body. It is important to avoid using sea water or vinegar to rinse the area, as this can actually worsen the jellyfish sting.
Immobilizing the arm and keeping it lower than the heart may be helpful in reducing swelling and pain, but it is not the most immediate action to take. Calling 9-1-1 or the designated emergency number should be done if the person experiences a severe allergic reaction, difficulty breathing, or if the sting covers a large area of the body.