meaning of "put on someone fifteen"

This appears to be a riff on the widely-believed phenomenon of the Freshman 15, which Wikipedia defines as "... an expression commonly used in the US that refers ... to an amount gained during a student's first year at college."

The concept of a "post-frogman 15" would then be 15 pounds that are gained by someone who retires from active frogman duty; a "frogman" being (again, according to wikipedia) "someone who is trained in scuba diving or swimming underwater in a tactical capacity that includes police or military work."

Professional-grade swimming is a notoriously calorie-intensive process (see, for example, this article on gold-medal swimmer Michael Phelps eating 12,000 calories a day—5 times the normal, recommended intake), so when a "hard-charging operator" (someone who is very dedicated to their police or military role and trains hard for it) "slows down" and retires from active diving duty (their status becomes "post-frogman", that is, no longer doing frogman work as part of their job), it is very difficult for them to avoid weight gain, as their eating habits may take some time to catch up to their new, much lower level of physical effort.