If she is don't like your 270 win for recoil, she won't like the recoil of the 257 either, plus the 257 will likely be a heavier rifle. I never have found any real difference in the recoil between them. And here is a link that shows the recoil is basicly the same between the 2 of them, plus the wby was in a 1 1/4 pound heavier rifle.
https://www.chuckhawks.com/recoil_table.htmI'm in the same boat with my GF, I tried to get her to use my 270 win, but she didn't like it, and she didn't like the 257 either, found the rifle to heavy. I been trying to get her something in 6.5 but so far she still using her 243 or my 240.
Frist you need to find a rifle that fits her right and what caliber's are available in that rifle, and if ammo is not that hard to find for that caliber.
I don't know how much less the recoil of the 6.5 PRC would be than your 270, likely not that much, but it might be a good choice. The 6.5-284 would be a decent choice, but rifles and ammo not easy to find. 7-08 a decent choice, rifles and ammo not too hard to find. The 6.5 creedmore, is really not that bad either, with 129-140's and keep it under 300-350 yards. The biggest thing with the creedmore, is that people are trying to shoot game with it 500-600 + yards, instead of keeping realistic hunting distance, just because they were shooting targets with it at longer distances. Or they was using ammo with a target bullet in it for hunting, and had piss poor bullet preformance on game. There is no reason why, a good hunting bullet in the 129-140 gr range out of a 6.5 creedmore, at reasonable hunting distance, won't work on big game, including elk. There had been millions of large game taken with the old 6.5 swed over the years across the world.
Or just keep the 243, it will work, chances are when she does get an elk tag, she will be hunting on your ranch, private land, where she will have lots of time, be able to pick her shots. It will be just like the moose she just shot, 100 gr bullet into the lungs, and your packing elk meat.
Rob