My mother in law is taking F on CX from HKG to SFO. Midnight flight. This is her first time out of Y.

I have never taken F on CX, but when I take F on SQ and the cabin has spare seats near mine, I ask if they can make the spare seat a separate bed for me instead of using my current seat.

Does CX generally allow this? Any particular etiquette other than waiting until all pax boarded and asking politely?

TIA!