Basically the BC is only responsible for the original structure but in this situation they would have to remove the alterations and return it to the original state. I’m sure the new owner would object to this so you could negotiate with him that the Strata will pay a proportion of the repair if he pays the rest AND a bylaw is registered passing responsibility of the new structure to the lot owner.
Who actually approved the work the EC or the BC at a GM. The EC doesn’t have the authority to approve improvements so that decision is meaningless but if a GM approved the alterations (and their are minutes to prove it) then you have a much weaker case and would have to pay more and the owner less.