Update guys this is what the previous owner has said to whispers new owner
Old owner: Careful about riding her till she has weight on...It will be difficult to support a rider right now with how thin she is
New owner: That is only the third time I've ridden her, the time at your place and one other time besides today,and it was only for a short walk about, yes she's got to get weight on but she also needs muscle and toning. Light riding will help her.
Old owner: Lunge work is more beneficial than weight on her back with her spine like that but yes, light exercise is great. You just don't want her burning too many calories. She looks like she's gained some weight though