I have a septic tank, and I have lived here for over 10 years. In my dealings over the years about it (it has not been too much hassle actually, but once my neighbour had her septic tank and her garden was filled with the contents so it is at the back of my mind), the nice people have told me again and again, that the problem is pouring fat down the drain. I most recently told them that I never pour fat down the drain but that I expect everybody says that. He nodded grimly.
I have sympathy in the sense that we were not taught this stuff at school, but perhaps you could show her this thread.
I pour my fat into a bag and into the bin but my mother didn't so I had to learn it and, thank goodness, not like my neighbour's hard and smelly way.
Does your neighbour know how much she is costing in the face of public advice.
Do Not Put Fat Down the Drain.
I think you did well to put this on mn because the answer is (forgiveably yet) Blindingly Obvious.