The most important question is whether they can see any paid options if they go into their own bookings.

If they can't, end of story: I think that they will then just have to pick up whatever's left when check-in opens.

If they can but they won't pay: same result.

The only reliable tip for "how to make that happen" is to pay for it, and to do so much further in advance than 48 hours before the flight, by which time FLY has taken over control.

Slightly more speculatively, if they can upgrade themselves to First, they might be able to pick seats in that cabin for free. But the upgrade is unlikely to be free, even if one is available.