I guess that I have 2 questions that might help find an answer:
1. what is the terrain like where she will be elk hunting? If she's in deep timber, a 150 yard shot might be the outside limit, so her 6mm would be fine. If she's out in the high plains of central Colorado, she might be looking at a minimum 250 or 300 yard shot. Whatever you choose, ask how will your cartridge perform at 300 yards. If it's heavy woods, choose a rifle that you can point and shoot quickly, b/c you won't have a lot of time to think about things.
2. Will she have a cow tag or a bull tag? If it's a bull tag, 6mm Rem and 243 Win are not ineffective, but have less wiggle room for error, especially if you find yourself "lobbing one in." If it's a cow tag, you could use either of those with more confidence.
I think you might already have enough tool for the job, but to make myself feel like I'm helpful, I'll add that I like these suggestions already given above: