From the responses you've gotten from people who have actual experience with taking elk with a .243, it appears both the 100 grain Partition and the 85 grain TSX are proven performers.
The only incidents anyone has offered on a .243 not working on elk have been due to poor shot placement (hind quarter or leg shot) that wouldn't be expected to work even with a much larger caliber bullet and shooting an elk that was moving as part of a herd. There was no description of shot placement on the moving elk, and I wouldn't be a bit surprised if the shot was too far back. A shot somewhere behind the lungs is again a recipe for failure unless one was shooting something on the order of a .50 BMG.
If you know the Partitions are accurate, then you know you have at least one good option. The rest is up to her practicing with the gun and being ready for the shot when you get her onto the elk.
