I’d also get an agenda item onto the next general meeting with a view to making a by-law that all quotations for work affecting common property include a separate section for the cost of disposing of waste and some kind of threat that if unsuitable material is washed down the drains they will be pursued to cover all costs etc etc. Then arrange for the bldg. mgr. to ask workers what the arrangement is; if they have no notion ask them to leave and get someone else. It would be more involved than this outline but avoiding unexpected sewage backflows is very high up on the priority list.
One of my neighbours had a similar problem caused by roots from an unidentified tree; I never go away for long without wondering whether they have now reached my (equally old and probably breached) pipes. Does anyone think I have any grounds to “demand” that a pre-emptive cleaning/check occur. I say demand because any form of reasonable request or offer to part pay is unlikely to get a favourable response.