Thank you so far for all your inputs.
My friend did the following: He was not able to book Mexicana online. He told me that there was a credit card problem, but he didn't know what exactly didn't work. So he went, here in Switzerland, to a local TA, which was able to book him on Mexican HAV-CUN, for US $ 285 one way. I don't know if this is a refundable ticket. In case of, he'd be able to get it refunded and buy a cheaper one in HAV when he arrives. But since he's not that much experienced with airtravel, he wants to be safe. Definitively not a flyertalker:-)