US Permanent Resident from India on Carnival Carribean cruise, need visa to entering Mexico Ports?

Friend is a citizen of India but is a Permanent resident of US, has Green card. He is taking the Carnival Cruise from Galveston, Texas which will stop at Cozumel Mexico and Progresso Mexico.

• Would he need anything else to take with him to be able to enter those mexican ports?
• For entering back in to the US, reading that Green card with Passport should be enough. Unless that was incorrect?

Thanks.