You don't believe them...do you believe your own eyes? Go see the videos. I had a friend with MS who was in much worse shape than Terri and we remained good friends, interacted, watched movies, went shopping (She in a reclining wheel chair). She indicated more than once to me that she was happy to be alive, despite it. She understood that life is a gift from God.
Your friemd with MS sounds like a fellow my husband was a caregiver for. He needed(needs) help to do everything. Yet, he likes to recite e-mails. He likes to talk on the phone as difficult as it is to understand him. He works out. All with help. Yet, if someone were to feel sorry for him, he would likely kick their butt.