Never. If doing a specific activity where it is interfering with breathing I might make a “suggestion” about gum. People w/medical conditions, on meds, trying not to smoke, older often chew gum.
In the water, I’d suggest spitting it out for safety. One senior I work with CONSTANTLY eats gum or hard candy, and he’s a retired dentist. If this makes the guy happy…and his breath is pleasant…at 85 who am I to reprimand or tell him don’t. I’m more concerned with the quality of his exercise/work out anyway.
I think it depends on what kind of workout/activity you have planned for your client. Tell a major league pitcher not to chew gum (or whatever!) and pitch and you’ll be laughed out of the park. I do think that in general the risks of choking outweigh the benefits of chewing gum in a typical fitness workout. You’re the trainer, you’re in charge.
As a general rule I would say no gum chewing during a workout. Just drink plenty of water!