Tenant trying to reduce rent due to a leak.

[Vincent Plant]

A few months back my tenant pointed out to me that the sink sprayer in the kitchen was at a drip. She didn't seem so concerned with it and I didn't see it as an immediate need so I admittedly brushed it off and have since forgotten it. 

So this month she shows up with her portion of the rent (shes section 8 and they pay most of her rent) and its $80 light. She hand wrote me a note stating that its reduced because her water bill over the past few months has been more because of this drip so shes taking it out of her rent! Am I insane or is this completely inexcusable? She can't just tell me what she's going to pay right? She calculated her average monthly water bill to show how she got to that number.   

I am a rookie landlord so I'm not exactly sure what to do here, and advice?