If the new water system is being attached to the outer wall, that requires a by-law. I would make the owner’s contribution (at least) to the removal of the lean-to a condition of granting the by-law they need to install the new water system on the CP wall.
That said, I’m not sure that the owners corp doesn’t have a responsibility to remove the asbestos from common property (the wall and the area on which the lean-to sits).