It makes no difference who put the Girl there.
The creator of the Bull has effectively had his work politicised. By all means remove the Bull entirely and replace it with something else. However, you can't politicise someone else's work without their consent, and then make him out to be the bad guy when he objects. It's a simple issue of respect as much as anything.
PS. For what's it's worth, I think artists do generally have legal rights protecting the integrity of their work from being tampered with in this way, so he may in fact have rights under the law that have been violated, but I have no idea whether New York/US enforces such rights.