Long Answer: While they can't edit the workout plan that you've assigned to them, they can change workout parameters, like, reps, sets, weight, rest, etc., when they're tracking the workouts.
For instance, if you've assigned the following exercise to them:
Cable Bent-over Leg Curl
Sets: 3 | Reps: 15 | Weight: 45 | Rest: 45 sec
When they start their workout, they can put in realistic values and update the workout logs. For instance, let's say they could complete just 2 sets (instead of 3), they could update it in their diary, while they track their workout. You'll be able to see that they could manage only 2 sets, and you can use that feedback to optimise their day-to-day training.