In addition to the questions suggested above, ask yourself:
1 - Will your friend appreciate the fact that she is a guest, abide by your family's rules and treat your mother with respect and gratitude?
2 - Is your friend a happy-go-lucky, go-with-the-flow kind of person, or someone apt to complain easily if a flight is delayed, a wait is long, etc.?
3 - Does your friend enjoy the same kinds of attractions you do (and can she handle them, or be patient if you want to do something that she has to sit out? (e.g., my DH can't do rollercoasters, but he's a great sport about waiting around for those of us that do). Can she handle hot, humid weather without feeling miserable?
4 - If you like to be present for rope drop, is your friend an early riser (or willing to become one temporarily)?
5 - Is your friend a "low maintenance" kind of gal? Does she show up for things on time? Not make a fuss if her hair is out of place? Never expect things to be "all about her" or create drama for drama's sake?
If you're unsure, I would suggest doing a little "trial run" before even asking your friend to come along. Have her accompany you to an amusement park, a local historic site or festival -- something that requires her to arrive early, walk a long distance, spend an entire day, and participate in collective decisions about what activities to do. Then you'll have some idea how she might conduct herself in Disney World.