1) Is he on a DL-operated flight?
2) Did be buy Premium Select, or did he buy Y and was upgraded to PS?
3) Have him look at the UG availability on the DL app. Not perfect, but will give an idea.
4) LAX>SYD can be a difficult UG, so if there are truly 6 seats available, but Z0, they're facing an uphill climb to both getting D1. Just guessing, but I'd put the odds at below 50/50.

As of this moment, current D1 availability is D2 and J4, so it looks like there are 6 open seats, assuming we're talking about DL41.