Hi Deb !! I have been to all 3 places a lot, and since your friend is not too keen about returning to Cozumel, I think you both would enjoy the Riviera Maya a little more than Cancun. As far as diving is concerned the Riviera Maya to me is by far better than the Cancun area!! Not only can you ocean dive you can also Cenote dive on the Riviera Maya. I have also seen and dived with Manatees in the ocean on the Riviera Maya, and played with wild monkeys in the jungle after a Cenote dive. To me i have truly enjoyed staying in the Riviera Maya area a lot more than the Cancun area.