if xray shows healed means fracture
has indeed healed .
what happens during three months of conservative care in cast is muscle gets atrophic and also the physiologic load bearing ability of bone was compromised . once fracture has united you should put her on a structured rehabilitation
programme where her tissues are progressively stressed and not suddenly overloaded .also fracture union site if pressed will remain tender for around a years time. so dont worry it will settle down and she will be fine soon .