Where in Cuba is he going?
I have fished there a number of times and caught quite a few tarpon, the biggest was about 70lbs.
I normally use an 11wt rod for my fishing but a 12 is also a good rod.
Most of the fish that I have seen in Cuba were small baby tarpon up to about 30-40lbs. These can be caught with a 9wt. however in May there will probably be a few larger migratory fish around which can get up to 100lbs plus.
Do not worry about the weight of the rod if you are sight fishing as you will only cast when you need to.
If you are fishing deeper water with sinking lines and blind casting then it is probably a good idea to go the gym for a bit of fitness training before the trip.