What could have stung my friend in Playa del Carmen?

I've had a run-in with jellyfish before, specifically last year around this time when I got stung.

The sting might resemble a bug bite, but it could also be caused by sea lice larvae, which are often mistaken for jellyfish. Soaking in hot water and applying topical cortisone cream can help alleviate the discomfort. Taking an antihistamine like Benadryl might also provide relief.