Can Rent be raised when adding Tenant to Lease in WI

[Pat Daniels]

We are in Wisconsin.  I have a tenant who allowed a friend to move in with out notifying us prior to the move.  Upon discovery we required that they complete and application and be added to the lease.  We also told them that the rent would increase by $50.  The current tenant if fighting the increase stating that she has a lease.  We had an attorney review and they see no rules in Wisconsin prohibiting an increasing rent considering we are issuing an addendum to the original lease with the increase.  Any insights on this would be appreciated as they have taken it to their attorney for review.