What is she looking for in a beach? just a sandy area to lay out in? walk-in water access? or will stairs do? "Cabana service"? i.e. wait staff on the beach for drinks and food?
AI's aren't our thing, but I agree, Occidental has a very nice stretch of beach, and is a very nice place overall. Adjoining "far south" AI's share the same stretch of beachline. It's long, flat and slopes right into the water naturally.
El Cid La Cieba has a nice beach, it is man made (like most beaches on Coz), and elevated from the ocean with stair access to the water. The property is much closer to town and is not an AI, although I think they offer it. Good snorkeling and shore diving. We used to stay there and I think it is one of the best locations on the island. Although some folks do not like the proximity to the cruise ship pier, we loved the people watching opportunities.
The Park Royal has a very nice beach area, although it's not a place I would necessarily recommend. Friends that ALWAYS stay at AI's have stayed there and were not that impressed, they enjoyed the Occidental Grand much more.
Casa del Mar used to have a small bit of beach accross the street from the property, but I don't know if the Park Royal took over it or not.
I thought Hotel Cozumel had the same situation, a small beach accross the street from the main property also.