Your mother most likely has a problem with her medial meniscus, (ie a tear), possibly with a secondary cyst, which may account for the swelling you describe. She obviously has had some previous biomechanical problems in her lower limb which led to the plantar fasciitis-in the same way she may have been causing excessive rotation through her knee in day -to day activites. If the tear is peripheral, it will heal with physiotherapy;if is deeper she may require arthroscopic surgery. Good luck.