Playa del Carmen cooking classes/school?
There's a fairly well-known cooking school in Merida:
but none in Playa or anywhere on the Riviera Maya coast that I know of. Merida is about 150 miles from Playa, so it's not a day-trip thing. But since your friends will be staying a while, it could be worth spending a few days there. Charming city as well.
You might try posting a query on locogringo.com. People on the forums are very knowledgeable about the area.
As nice as Playa del Carmen is.... getting to that part of world & not exploring all of the Yucatan would be a crime. A week or two in Merida would be good for the cooking classes & as a base for exploring... Tabasco, Campeche, Uxmal & the Puuc Route, Izamal & other largely undiscovered Mayan sites near Merida.
This is a real long shot but you might check with Guidos on Cozumel (a short ferry ride from Playa del Carmen - really it's the closest other 'town' of note). If I recall, Guidos was the place run by a couple who were both European chefs who bailed for life in Mexico. (This was in the mid-90s - hopefully it's still them, there.)
If anybody would know about cooking schools in the area, I would think it would be them.