It strikes me that even if the fence isn’t common property, if the fire exit is required then that would be CP and it’s up to the owners corp to pay. I’m sure someone will start quoting chapter and verse of who owns which part of the fence – cue My Pink Half of the Drainpipe – but that’s irrelevant if this is adjudged to be a necessary fire safety measure.. However, if it isn‘t a necessary fire safety measure, then Mum could be paying for this on her own (if she can get the neighbours’ permission to put a gate in there.