House hack plumbing expense, who is responsible for the cost?

[Mariah Porter]

I am currently house hacking my first property. I rent out two bedrooms to roommates and live in one of the other bedrooms. I just had a sewage line blockage that caused a backup into on of my basement bedrooms. I had to rip out the carpet and padding and might have to replace all the baseboards and possibly some drywall. The plumbers had to excavate down to the sewer line and get the blockage out. It was really big and they couldn't do it with a snake.

The cost for the plumbers is $1500 and they actually gave me a discount because they just barely replaced my sewer line for me a month ago. The cause of the blockage was tampons flushed down the toilet catching on the pipe and then other things catching on the tampons etc. I personally have not flushed any tampons down the toilet. I asked my roommates if they did and one of them said that she hasn't and the other one said, "I did flush a couple." This roommate ended up moving out a couple of weeks ago. She broke her contract so she still had to pay me rent for Jan, which she has paid. I still have her security deposit and feel justified keeping it and putting it towards the plumbing expense but it's only $600.

Because we're all living there, I can't really prove that it was entirely her doing and the other one isn't lying or mistaken about not flushing any tampons. So I'm not really sure what to do about the bill. In my lease it states:

"Tenant shall be responsible for damages caused by her negligence... of both the leased room and common areas subject to tenants use. Tenant shall report all building damage, water leaks, or other maintenance issues immediately to Landlord or will be held liable for the costs of repairing any unreported damage. If the need to repair is caused by Tenant...  Landlord may make the necessary repairs and the cost of which will be treated as additional rent to be paid by the Tenant upon notification of amount. Failure to pay costs of repairs will be treated as additional rent payable by Tenant and due immediately."

Can I bill her for the cost of the repair? 

Have any of you faced any issues like this in a house hack?

How would you deal with it?

Thanks!