Y class to Sydney is a bloody long way no matter what size you are. To get back to the original question, two seats are certainly preferable but it would be cheaper to book one J class if you are using miles.

If you are certain that you are booking one Y class seat with miles then I suggest the following:

1) Do a stopover to get a good nights sleep. (United in LAX, Air Pacific QF in Fiji, Air Canada in Honolulu)

2) Contact csa and have notified on passenger record that your Bro is a good natured BIG Aussie

3) Request advanced seat assignment for at least a Exit row seat but preferably an exit row seat with extra leg room (depends on aircraft type)

4) Request some lounge passes for your bro from fellow flyertalkers. (depends on airline and generosity of FTers - If Air Canada is your airline from New York to Sydney with stopover in Honolulu using United points then contact me!)

Cheers,
Dave - London