How to find the water connection behind a shower wall?
What is on the other side of the wall?  Cutting a hole in the drywall on the other side would be the preferred method.  If the 2" diameter hole is not large enough for both flex-lines, the you might need to enlarge it, or drill a 2nd hole through the tile and backerboard with an appropriate bit and lots of muscle.

Though.... if the contractor abandoned the job the tile job probably wasn't done right either and will leak meaning you will need to rip it down anyway.