Landlord-Tenant issue

[Kris Fox]

I'm sure something like this has been covered before, but here goes...

There was a plumbing issue with the main drain line. There was some gurgling when the sinks/tubs would drain.  That's obviously not normal.  I called the plumber. It turns out there was a lot of debris in the drain between the house and the city side of the sewer.  This was the first tenant after a major rehab, so I gave the tenant the benefit of the doubt and had it completely cleaned out.  

Fast forward two months, I get a call that sewage was coming into the tub.  Same plumber goes out at 9 pm.  He swears up and down that there is NO way that this was residual matter from before.  Baby wipes, tampons, prophylactics, etc.  This was all material that occurred in last two months.  Tenant swears up and down that this is not there's. They get into a screaming match in the middle of the road at 12 am. Cops get called.  I just received a $600 bill from the plumber.  So the questions...

#1)  Is there anyway to deal with the tenant without calling them a liar?

#2) Would you foot the entire bill with the tenant?

#3) How do you deal with a plumber that isn't professional enough to keep his cool when he's dealing with an irate tenant?  Should this affect the amount of money they receive?

Thanks a lot, BP.