I would start small & work your way up. Check in with her after the first to see what she did or didn't like. That will give you an indication of what to steer clear of. For instance if she says it was too jerky, or she's a little motion sick, you'll know to stay away from those types. I'd start with Splash Mountain. It is mostly a slow moving, level log ride. There is 1 large drop that you see from the walkway (even though it looks bigger from the walkway than it actually feels in the boat), & 1 much smaller "bump" drop at the end.
These are some characteristics of some rides...
Big Thunder Mountain - Probably my favorite ride. A little jerky, lots of tight turns, only minor-medium drops.
Space Mountain - all in the dark, very jerky, seating is 1 behind the other, drops feel bigger because of the dark, speed feels faster because of the dark.
Mission Space - you are fully enclosed inside a capsule with a body var over your shoulders locking you in like RNR. The capsule moves with the movie, the intensity will vary depending upon the level you choose. Although the screen is very small (probably 12"), this is a simulator ride & may trigger those who are sensitive to motion sickness.
Test track - jerky only in 1 spot. Fast, open air ride going around a race track a few times.
Tower of terror - mostly in the dark drop ride. Very jerky. You will feel like you are coming out of your seat briefly.
Barnstormer - very tame roller coaster, pretty fun for a little kid ride. Seas are very bumpy & your back will hurt from them after.
Primeval whirl - pretty tame, but lots of quick turns & spins (like a teacup) so it can trigger those sensitive to motion sickness.
Expedition Everest - there is 1 climb that seems very high & you there is nothing on either side of you. The actual drops & turns are medium. The ride will seem fast speed at times. There are portions in the dark & backwards that give you the feeling that you are going to go upside down, though you don't.
Seven dwarfs - short roller coaster with a handful of sharp turns & a few minor-medium drops. Pretty smooth ride & enjoyable, yet quick.
There are many others that are more rides than rollercoasters. If you think motion sickness or back issues might play a role, we can provide more info on those.