Late Fees for Utilities?

[Coleman Nelson]

We have a lease that stipulates rent is $525 per month and the tenant is responsible for all utilities.  This is a duplex with one water meter, and we had submeters installed to divide the cost between the tenants, and we bill the tenants.  One tenant does not like that we have included the water bill with their rent payment and make both due on the 1st, since the lease does not mention the water bill specifically.  They do not believe they should be charged a late fee if they pay their rent on time, but not the water by the 5th of the month.  They have paid on time so far, but it could be an issue in the future.

Should the water bill be subject to the late fee as well, or only the rent?  Is there an industry standard for the length of time between getting the water bill and the due date for the tenant to pay?