The ability for a body part to move requires multiple organ systems to include bone, muscle tendon and nerve. The inability to flex the foot suggests some type of disruption in this process. This may result from muscle tendon pathology, neurologic failure to promote a muscle contraction or a combination thereof. Given the limited information provided by the question I would suggest that more than likely there is no neurologic component to this as you apparently also suggest pain but this is not guaranteed. More than likely failure is occurring through the musculoskeletal system and may result from overuse processes to produce tendinitis which can weaken function and produce pain when recruited. Depending chronicity of the disorder treatment and prognosis varies. In acute cases appropriate rest, ice and anti-inflammatory may be all that is required. In chronic cases, mitigating the factors that have caused the overuse process is required to include activity modification, addressing any biomechanical issues and physical medicine rehabilitation. Given that this is a recurring consistent problem I would recommend evaluation by a medical specialist who can more appropriately evaluate her presentation and obtain require test such as x-rays that may help define this process.