What excersises can a person do with really bad case of multiple sclerosis?

See physiatrist. A physiatrist is a doctor of physical medicine and rehabilitation. Their focus is on maximizing the physical function of every patient, within the context of their current impairments. An individualized treatment plan will be developed in conjunction with a team including physical therapy and perhaps occupational therapy.
Tolerability. Walk, stretch, few minutes at a time. Listen to your body and stop when fatigue starts. Don't push it. Have a physiotherapist evaluate you.
Depends. It depends on what body parts still work. Can you stand? Walk? Are you wheel-chair-bound? Can you move your arms and legs? There are yoga exercises and strength exercises that can be done in a wheelchair. There are aquatic exercise classes that may be helpful. It really depends on what abilities you have left.
May be tough. If possible, aerobic exercise is helpful. Maybe supervised pool exercise, but stationary bike, flat treadmill holding onto bar, repetitive calisthenics, might work. Personal trainer in physical therapy clinic best choice to proceed. If bad case, is patient taking potent medication such as Gilenya or Tysabri (natalizumab)???