Grow medical marijuana and start a mail order/storefront business.
Assuming he can get the proper permits, add the right lights, and roof the place with class or something else to get the sunlight in. May also need security to keep someone from stealing the crop of Marijuana.
Not sure what sort of humidity needs to be maintained to get correct and predictable quality levels also not sure if you should allow insects in to pollinate or not, I never had an interest on it. One thing is growing for you and another for commercial distribution where active levels need be known and quality maintained, lab needs to verify active ingredients from time to time.
As for personal medicinal use, I think I read you can have up to 5 plants, but I could have misread, check don't get arrested.
Have not heard anything about growing Coca leaves in US or PR, but if there is some legal way to do that, it is also a way to get high price crops. Don't smoke the profits.