I see you've already accepted an answer, but in case anyone else reads this and doesn't have the budget for a suitable bedrail, cheaper alternatives are:
- Buy a couple of swimming pool noodles and put them along the edges of the bed under the sheet. The sheet does need to be a fitted one (and fairly tight fitting) to hold them in place but it works well.
- Put pillows or a rolled up duvet on the floor - rather than preventing her falling out in the first place, it simply ensures she doesn't hurt herself if she does fall out