Looks like a decision that has to be based on lots of information. The gentleman may not be in pain, but is he in danger? Might the joint collapse in some way if not seen to? The main risk would be in the actual surgical intervention, time under anaesthetic, possibility of infection etc. At 90, if he is hail and hearty, it may be that living with the damaged hip could be an easier option. Not something that can really be answered without considerable expertise, but good luck anyway.