Eating correctly can mean a number of things: Healthy/Clean, to manage existing health conditions, to the point of satiety or a bit more than usual some days.
How much and enough is more pertinent. Maybe suggest eating/drinking a bit more. Maybe a glass or milk in the morning and before bed. It looks like this program puts him nowhere near Maximum Recoverable Volume where it’s so much literally nothing you do will allow you to fully recover so his muscles should be able to recover at least.
Sounds like he is concerned about joint issues from his training as opposed to muscle soreness. Sounds like his joints are getting hammered by the heavy weights. Reducing volume may improve how things are going but maybe some other changes could be made.
Due to his age you’d expect degenerative changes in his joints such as knee osteoarthritis
and in the spine: disk herniation or degeneration or spine osteoarthritis. As above poster mentioned given the high rep BB Rows maybe spinal erector soreness or strain could be an issue.
Exercises are great for the general population but maybe consider how you are loading joints, the pros and cons and the sustainability of such. If there is already degenerative processes going on lifting at best won’t make it worse however that doesn’t seem to be the case.
In the Squat shear and compressive forces load up the spine and obviously the knees have their share of load. OHP stresses the spine. Same with DL and BB row. Almost all movements in the program stress are taxing on the joints. Sometimes it’s needed e.g. the squat being such a useful movement but are all the exercises absolutely necessary?
Is substitution of exercises possible? If not maybe lower volume on the most stressful of movements e.g. DL or BB Row. I’d take the decreased training stimulus over not being able to sustain training long term ez.