there is a customer here that does basically low weight but 6 sets of 40-50 reps. He is 70 years old. Is this safe?

It is difficult to make a call without seeing the performance of the exercises. It is likely that he is neither doing harm nor improving fitness. I imagine there is some chance of repetitive stress, though that would be more likely if he were doing these movements on a daily basis. Which he may be, but we don’t have that information either.

I would want to know if he is lifting to VMA/failure. If he was moving quickly or slowly, controlled or not controlled, ROM used, overall mechanics both postural and in the movement, and possibly mental aspects like OCD, TBI, psychological issues, etc.

Does he appear to be in pain at any time? Have you observed him during non exercise movement?

I would be hesitant to approach a person in this type of situation without a bit more to go on. And if I were thinking about bring this up, I would invest quite a bit of time and effort to become acquainted with the person. I would want to know them better and begin establishing rapport to lay the ground work for any type of intervention into their routine. Often, just in the natural course of connecting with patrons and attempting to show that I am approachable, patrons will bring up the subject of what I think of their routines.

If do decide to become involved, be prepared to back off if the person appears to become defensive. You won’t do anyone any good if you press them and they judge you to be critical or too pushy.