I agree about using a premium grade bullet in the 243, it should help on odd angle shots and where you may need some extra penetration. Not all shots are simple side into the ribs shots. A friends wife uses a 243 with Nosler Partitions for elk, has for 20-some years I believe, without a lost critter.
I also agree with the general sentiment that IF the bullet goes right where you want it, it isnt generally a bullet failure, however, if a 243 was all that anyone really needed for elk, people wouldn't use larger calibers, they wouldnt have gained the reputaion they have for heavier animals, and the premium bullet makers would have never got off the ground. Heavier calibers can help, as can better grade bullets. I believe there is such a thing as bullet failure, for certain shots, and heavier animals in particular. Smaller caliber, relatively light weight, standard cup and core bullets can break up and/or fail to penetrate as well as desired when they hit heavy bones.
Another thought, IF you have a truly good gunsmith available, is rebarrel the 243 to 7-08 or 308. Factory take-off barrels can often be had for $50-$75. It would need to be fitted and headspaced, it may need to be set back a turn and rechambered, or perhaps not. She could then keep the stock she likes, and the gun would be familiar in al other respects. You could then sell the 243 barrel to recoup some of the expense.
"Far better it is to dare mighty things, to win glorious triumphs even though checkered by failure, than to rank with those poor spirits who neither enjoy nor suffer much because they live in the gray twilight that knows neither victory nor defeat." -Theodore Roosevelt-