Continuous clog in the sewage line - whose fault?

[Christina Tkacs]

Long story short... our tenant has been in his unit for approximately four months and has reported a greywater backup every month in the septic system (while running laundry, backup occurs in bathtub).

We've spent over $6,000 for the sewage company to bring cameras out (4x) and clear clogs.  The only thing they can find is a piece of rebar through the line, underneath the foundation.  This unit has been in existence since 1974 and the previous tenants (~15 years in the unit) had no problems.

I don't know what to do.  The sewage company suggests ripping up the foundation between the units and taking out the rebar, but they're not positive the problem will be fixed ($10,000 repair).  We also can't tell the tenant to deal with the graywater backup.  We also can't have the sewage company come out and keep unclogging it.

I don't know what to do.  HELP, please!  We're at an impasse right now.