My mom has a fear of heights and doesn't enjoy the falling sensation. We have been to WDW enough times there isn't anything she has not tried before and she will ride anything. She still doesn't like TOT or the drop on Splash Mt, but she reminds herself that the overall experience outweighs everything else. But she definately did not start out on these rides!
Her favorite has been Space Mt. because it is a dark ride. She know's she is high up, but she can't see how far. This holds true for RNR, now her favorite ride. It's dark and fast so she doesn't have that falling sensation in the loops. Now if I show her a similar ride in daylight she refuses to go anywhere near it. She also loves BTMR. Yes there are two steep inclines but you don't fall straight down.
My suggestion would be finding videos on
Youtube of the rides, such as Space Mt (with the lights off), BTMR, or any ride you think the height might be an issue and have her watch them. Also watching BTMR in person from the walkway can be helpful.
There are plenty of things to do where heights are not going to be an issue. I just know for my mother she likes to be locked in (another reason she doesn't like the two drops in Splash Mt) and prefers dark rides.
If your mom is intersted in trying a ride, I would suggest Space Mt. or BTMR to start. Not to mention they are over VERY quickly regardless.
I hope that helps!