Best exercise for someone in a wheelchair?

My Mom has been diagnosed with probable MS and has difficulty walking. She can stand a bit and walk with much effort, but not very far. Can anyone recommend exercises or equipment that would be appropriate for someone that is mostly in a chair? She’s gained some weight in her midsection and so exercising her arms, although maybe helpful for aerobic benefit and some calorie burn, isn’t the full solution. Lifting her legs is difficult, so getting into a recumbant bike would also be too hard. Thanks for any suggestions!