It may depend on a few different factors, but I think 6 sets of
40-50 reps could be setting the person up for overuse–given his age and any orthopedic issues he may have. It also depends on how many days a week he is doing this. One question to ask and think about is what benefit is he getting with that many reps? With my older clients I generally use a rep range of about 20 max (of course it depends on many factors related to each individual).
Here are some good guidelines from ACSM on resistance training and the older adult:
Hope this helps you.