It's a quick 5 story drop. It only lasts a quick second. Splash Mountain is my 75 year old Mothers favorite Disney Attraction. She's definately NOT a ride person, either. My son has been doing Splash since he was 6.
Unless she is very, very afraid of heights/drops - she will absolutely love this attraction.
If she does decide to go on it, talk her thru it. She may be so nervous worrying about the Big Drop, that she doesn't enjoy the whole story beforehand. Also, let her know there are a couple Practice Drops inside to help her get ready for the big one at the end.
I think alot of how our children handle rides depends on how we handle them ourselves & what emotion we show them. Help guide her into the excitement, rather than anxiety & she'll do great.