Thanks for the quick respond. It is Flex fare. Unless there is a plane change, the one he is on now does show J on the A319.
Chances should be fairly good. He will have to keep in mind that if the return flight is not upgraded prior to ckin, the ckin staff at CUN sell off the upgrades for $250 a piece at the airport so he should be aware of those competing for the space. The earlier at ckin the better chance for upgrade in my experience and this has been as an SE.