I don' think he was ripped off at all. Taxi drivers are not banks. Like all businesses in Cozumel. They can set their own exchange rates for US dollars and in this case it was 10:1 which is a very common rate. Any exchange rate issue can be easily solved by paying in pesos. I think you just like to whine!
I don't think that is fair. At one point the peso was 14 to the dollar. At 10 to 1 that is a considerable difference. Sure I agree that pesos are the way to go always but they can collect dollars at 10 to 1 and spend them at Mega for over 12 to 1. Speaking of Mega, they post a nice big reasonable exchange rate out front. I also tend to check the exchange rate at restaurants when they post in dollars and pesos. I can't say any of my favorite places or any I remember tried to pull a 10 to 1 rate. In fact sometime their rates actually made it cheaper in dollars. 10 to 1 is, in my opinion, shaving the tourists. To say they can pick their own exchange rate is to say they can charge what they want. By that they could say 8 to1 or 5 to 1. I leave it to your judgement whether that is on the beam or not.
Again though, I load up on pesos at the BOA cuz I like to do like the Romans you know. Maybe that was your point? Get Pesos? I agree with that. Oh and I hardly ever take a cab. And when I do, 3/4 of the time the fare is ok and I tip.