Treatment For Burning Rashes On Leg After An Insect Bite

age 15 weight 125 height 5' 5'. yesturday at the beach at around 4 p.m. my friend was bitten (this is his information by the way) by some unknown sea creature near the shore. He says that he had uncontrollable pain up his leg, "as the poison traveled up his leg." he told a lifeguard and he was given vinegar which according to him had no effect. When he woke up this morning he had a rash that looks somewhat like a burn. I was wondering if this is serious ? and what he should do about it?