Hi
Have you checked strength (L) versus (R) hammies with dynamometer?
Pain pattern sounds like it might be hamstring origin tendinopathy/partial tear or central tendon tear- slow to heal like most tendinopathies and aggravated by aggresive stretching - could confirm with ultrasound diagnosis?
If so she probably should stop stretching it and focus on hammy strength training(eccentric/concentric inner and outer ROM)