You say the tenant caused the clog but it isn’t as simple as that. What did the tenant do to cause the clog? If they used the toilet as one normally would they have done nothing negligent and as such, not liable for damages resulting from the clog. If they stuffed a baby blanket down the toilet they’re liable for everything.
Toilets running aren’t always noticed. They can run quite quietly and wouldn’t be noticed unless one happened to see water movement in the toilet when initiating use....
but not too many people really pay that much attention to the water in their toilet.
I notice you said unclogged/repaired the drain. If repair refers to anything other than using a plunger or maybe a snake to dislodge a clog, there is a good possibility there was an issue within the sewer line that caused the clog. It generally takes a lot to clog a sewer drain. A large diameter pipe is used all the way from the toilet to the final connection in the street or septic tank. More often clogs such as this are when homeowners discover they have a collapsed line or there are root intrusions.
So, what did they do that was negligent that caused the clog?

