Seems like fasting would be detrimental to his workouts.
Depends on the type of fasting really. If it is a full four day fast, then combined with workouts he needs to take care. but I am sure his professional trainer will help with that.
If it is one or two days of full fasting, not a problem. Again, the trainer.
Finally, if it is intermittent fasting, then no real problem at all. I do this sort of thing.