I personally dont agree with homing white cats as indoor only, I have had 2, both came to me as seniors with no issues with their ears, and Mimi had been fed at my neighbours work for 18 months and her ears were fine, despite very thin fur (so her new owners got a warning), but if he is deaf as well, she needs to realise the danger to letting him roam free, i would tell her that you are concerned he will have a serious accident as he can't hear things, and therefore can't get out of the way, especially with an anti-cat dog next door, it can be difficult for 'able' cats to get out of a dogs way, never mind one that wont be able to hear it.