Please keep refeeding syndrome in mind, and design a diet to avoid it. If you can avoid refeeding syndrome you should be fine.
The lordosis isn't as concerning to me as her body condition. Yes she probably can be ridden once she recovers. I'm not certain i would put an adult on her, but if she's exceptionally calm, she will probably be good for small children. Maybe have an adult test ride her a couple times first.
You want to consider whether or not she has arthritis or lameness issues before deciding on riding.
First get her healthy! If you feed her the right diet, she may make a full recovery. My Paso Fino was badly starved and he bounced back quickly.
Diet to prevent refeeding syndrome:
Days 1-3: Feed one pound (approximately 1/6 flake) of high-quality alfalfa every four hours (total of six pounds per day in six feedings). Contact a veterinarian to evaluate the medical status of the horse. Note: A scale is essential for weighing the hay in the initial steps of refeeding.
Days 4-10: Slowly increase the amount of alfalfa and decrease the number of feedings so that by Day 6 you are feeding just over four pounds of hay every eight hours (total of 13 pounds per day in three feedings).
Day 10 and for several months: Feed as much alfalfa as the horse will eat and decrease feeding to twice a day. Provide access to a salt block. Do not feed grain, treats such as apples or carrots, or other supplements until the horse is well along in its recovery. Each feeding of grain, treats or any supplements complicates the return of normal metabolic function and can result in death.