Mother in law is moving in and we are hiring her as a nanny.

I appreciate all the advice.Have the sewer lines scoped with a camera. 

Should i hire a plumbing company to send a camera down the main sewer line?

Whatever it is you have to get a qualified company with a camera to figure out what the issue is.

There are wireless cameras available on Amazon.

Also the city likes to pick on the landlords of these properties that the tenants destroy to make themselves look good in front of the camera.    

Try searching on the Boulder Camera's web site and I think you'll find details. 

If he actually complains he doesn't have a picture/video of that ask him why not, since everyone has cameras on their cell phones these days and if your contractor was using that much water there should have been plenty of time to get pictures/video of it.

I also walked through with a DSLR camera and took pictures of everything that I would send to anyone who asked.

My last house & this one is a fortress, lit up like a bank at night, alarm system, cameras all over & me heavily armed inside.