KP is right. The Owners Corp is reposnible for any changes made to Common Property prior to the current owners’ purchase – but they could demand that the current owner allows them to reinstate Common Property at their expense (an unlikely scenario but a bargaining chip, nonetheless).
However, it is possible that his interior wall had little bearing (no pun intended) on common property, in which case the question is moot. However, it would be worth opening negotiations on an amicable basis to check if this was a load-bearing wall and what might need to be done to make sure there are no problems in the future. If it wasn’t load-bearing or if it was and adequate supports were installed then I would just let it go.