I agree with Veronica, start her standing when she is riding alone (or with the coach).

Since you say she can already stand on the climbs, my guess is that she is comfortable standing when her cadence has dropped, but not when she's spinning at 110+rpms.

Here are a few ideas for drills:

On long flat sections of roads, stand and coast, just to get comfortable standing on the bike. Practice standing on the hood and in the drops.

Find a long ~slight~ up hill, one that you would normally NOT stand on, but that would require gearing down a bit. Instead of gearing down have her choose "too large" a gear. She'll have to stand. Again practice standing in both hoods and drops(standing in the drops is important for that final sprint).

Do "kilo starts". Come to a stop (if she can trackstand) or "almost stop", but instead of gearing down, again gear up, using the large chainring up front (this is also a great workout for building power BTW). Have her take off (she'll be standing) trying to reach full speed as quickly as possible. No sitting allowed.

Practice standing and spinning indoors on the trainer.

If she is afraid of the bumping that happens in a bunch sprint, it's time to take the group to a grassy field for bumping practice.

Later, on group rides, she should be encouraged to stand as much as possible. Try standing out of every corner. Sprinting for every city-limit sign.