How much can a shareholder in a coop charge a roommate?
An apartment owned and occupied by a shareholder/lessee in a non government regulated cooperative (for example: not a Mitchell-Lama Coop, not an H.F.D.C. Coop) is outside the scope of rent regulation and would not be rent stabilized. In the free-market, a roommate pays whatever rent is agreed to by and between the roommate and the shareholder/lessee. There is no mandate that the roommate is charged up to a so-called "proportionate rent" as one would expect in a rent stabilized apartment.
So if you are certain that the building is a cooperative, that the lessee is a shareholder in the cooperative and that he lives in the apartment, then it is not likely that there is any limit on the rent imposed by law, code or regulation.
The number of "roommate" is regulated by law; and the cooperative likely enforces the restrictions on the number of roommates. The general rule derived from the statute is that one tenant is allowed up to one roommate and that roommate's dependents. If there are two tenants on the lease, there is no roommate allowance unless one were to permanently vacate. That is not likely in a cooperative. So if there is one tenant/shareholder/lessee the statutory right is one roommate. The cooperative likely has a way to deal with shareholders who have an extra roommates.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.