I was told by one of the owners of the unit below me that there was a water leak and was show some photos. I asked to see it and got some answer about it being closed up. We made an arrangement to use their "hand dolly". We pulled the up/down unit out enough to check, running a small spin. It was a pinhole leak in the hose the installer had used.
I don't deny that it came from my washing machine. I offered to call by son-in-law. He fixed it when the shower leaked. In the several cases at this condo. The unit above fixes and pays for the damage. I was willing to do this. The owner started getting bids saying she would do it. I assumed it was for the many hours I had spent fixing other probems. That happened end of Nov. Feb 12th, I received a letter and a bill cfor $900.

Submit it to your insurance company and tell them about the installers error. You may feel that she should have let your son-in-law do the work but she is not obligated and is entitled to professional repairs