Is this the plumber's fault?

[Zoe Lee]

At the end of 2019, our tenants reported clogged toilets. We sent a plumber to investigate. He recommended replacing a sewer line. We authorized the work. A week later the tenants' apartments got sewer water backed up into the bathroom. 

Since the property runs on a septic system, currently we have a septic company look at it, they found that the plumber didn't replace the entire sewer line as he promised. He was 4 feet short. 

Granted the septic system has other problems too, but never sewer backup. It was only occasional clogging. 

I'm not sure if his work caused the backup. But the fact that these two units with his replaced line got sewer water backup, not the other two which are on a different sewer line, and the one-week time frame makes me suspect it might be something he did. But I cannot prove it.

Now the plumber is demanding payment after we spent 15k repairing water damage and vacating tenants. 

I feel that he should take some responsibilities but how do I go about it?

People in this forum have been holding our hands navigating some tough issues since we stepped into the landlord role. I'm very grateful and hope someone can shed some light on this issue too. 

Best,

Zoe