so you want to put your own fence up... but going halves is a no-no?
that makes no sense ... one way you pay full price, the other half price ...
you also ask if you can put something up to stop the animals escaping - then when told, yes you can, you say you can't because you are frightened of a backlash!
make your mind up! what exactly are you expecting people here to say to you?
you don't owe her anything, no... but she owes you nothing either.
basically if you want a fence there you have to do it yourself