I'm not a lawyer, but a common person would expect the property owner is directly responsible for damages to adjacent property caused by equipment failure installed on his property. Secondary claims to manufacturers or other third parties is at the discretion of the owner and not pertinent to primary claims against the owner. In other words, sue the owner. If the owner wants to sue the manufacturer or plumber, that's his business, not yours. Ownership documents may extend or alter common legal expectations.
Fuel burning appliances must not be installed in sleeping rooms, with the exception of direct vent appliances which draw combustion air from outside.
Relief pipes must lead outside of the building, their outlets no more than 2' above grade, no less than 6" above grade, and be directed downwards. There is no requirement for the pipe to be attractive.
Even if it were permissible to drain into a DWV system, it would have been required to be via an air gap and receptor which would have prevented drainage water from entering the boiler.
The local building authority would be mandated by law to ensure buildings and related equipment be installed in conformance with governing codes. They are not all seeing and need to be notified of work being done to property to ensure compliance. While they would like to hear about non-complying work, their ability to respond to specific complaints may be limited by time and budget constraints. In other words, do complain, but don't expect a response. It's not possible to have a third party inspect property without the owner's consent. Only government agencies could possibly have this right.
In this particular situation, unless precluded by ownership documents, the owner suffering damage from the unit above should be compensated by unit above owner. Legal action may be required to achieve this. Relief pipe should be extended close to grade and be directed downwards. Complain to your local building department if you think it'll help. Unless you know the details of the combustion air provision, you may have to let the installation in a sleeping room pass.
All of this is USA oriented, but something similar would be expected nearly anywhere.