The Mist Trail is much steeper than the JMT, and there are sections of steps that are fairly high (over 10" steps), so this would be the most difficult part of the hike for your fiancée. If the JMT is open by the time of your hike I would certainly recommend using the JMT over the mist trail. Also, comming down the mist trail can be a test of your knees, so many folks (including myself) go up the mist trail, and down the JMT.
You might consider a "training" program for your fiancée. You do have a couple of weeks, and starting with shorter hikes, and building distance and climbing might prove easier than you might think. Even your feet will "get into shape" by walking, which will make the experience much more enjoyable. She should wear whatever shoes or boots your she intends to wear on the hike. Start now (if you have not already begun).
