He can easily dial down the Workout Intensity if he finds the VO2 Max (or any other workout) too much to handle. It is a great tool and is something that should be considered by more riders.
The 120% VO2 Max value used in many of the workouts is a find approximation that works for many rider. But many others (especially Masters+ riders) can find them too taxing. I recommend rolling down to around 95% Workout Intensity if there are concerns.
Here is a chart that shows VO2 Max is around 105%-15% of FTP.
Here is a chart I made that shows the impact of changing Workout Intensity, and how it relates to the 120% default for VO2 Max.
As long as a person keeps Workout Intensity around 90% or higher, they are still in the VO2 Max realm. So it is an easy thing to adjust on the fly and then increase over time as he gets more capability.
