Holy crap! (no pun intended) So, it is possible?
So, does incoming water use those same pipes, too? I doubt it. Aren't they under pressure, and/or a distinct separate part of the plumbing system?
I don't do civil litigation, and property law was never a specialty/interest of mine, but this should be a condition to break the lease, I would imagine. My friend is super-clean, like OCD clean. She couldn't even sleep at her place last night. Oh, and yes, it was high pressure air blast that was used.