I think it largely depends - but more on the rabbit than the cat. Wild rabbits are much more at risk as they act much more instinctively as prey animals whereas domesticated ones will generally feel a lot more secure. Ironically, I reckon it's the instinctive panic and running that puts them more at risk - so nervy domesticated bunnies may have a similar problem. My own bunnies will all quite happily chase cats off, even cats which are real 'killers' in other ways. It seems to really disconcert most cats when a bun doesn't respond in the way they expect prey to, and it often catches them by surprise and the cats turn tail and run. One of mine is a 900g nethie and she is exactly the same so it's not so much size, as disposition.
Once when we had new neighbours, their cat spent the first few weeks staring out of the window at the buns in our garden, then when they first let it out, it was with fear and trepidation that it might harm the buns. They knew their fears were unfounded when my two at the time triangulated on it and got it cornered
Although say it's unlikely that these cats will harm your neighbours bun, I wouldn't rule it out completely, and if it's stressing the bun out then it's definitely best avoided anyway - stress can cause rabbits to stop eating and because of the way their guts work, that can be fatal very quickly.
