[dra&pigs]

I would show them this site. The boars are probably not friendly towards each other because of her. If they are keeping her in the same room as them, they are most likely fighting over her. Removing her from the room would probably solve the problem, especially if they have not fought before her arrival. You are right-nuetering one of them would not change behavior as it does in other animals, and they cannot have both nuetered and placed with the girl-they will most likely fight (I have never heard of an instance where this is not the case).
Again, I suggest showing them this site. They can read through the anti-breeding and nuetering posts on their own and find out the correct information.