Tenant wants rent reduction due to increased toilet flushing

[Connor Brokenshire]

Long story short, we had a sewage backup in the basement at one of my properties. The plumber came out and fixed it and recommended that the tenant flushes twice on their solids due to the pipes being older. Tenant is now requesting a reduction in rent since they pay the water bill and are now being told to flush more than normal.  Has anyone run into a situation like this and what would you recommend?

Thanks in advance!