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?

Hello M.C. Doreen Rioux,
Is this person retired from work and/or has much free time? It should be fine. This is no different from a stock person loading shelves eight hours or more a day. This works muscle endurance. If the client seems comfortable, I would have no worries; although, I do not know this client’s history which may say otherwise.
Take care,
Natalie aka NAPS 2 B Fit.