Hi John Morgan, what a great friend you are to be so concerned and want to help!
The best thing you can do is to be a good listener. Find out why she self-isolates. Is she in pain? Weak and afraid she'll fall? Does she suffer from a "foggy brain" and afraid she'll embarrass herself trying to talk with people? Is she incontinent and afraid of having an accident? Is fatigue keeping her at home? Is she having trouble sleeping? You've probably already done this, but the next thing would be to ask what you can do to help.
She won't venture out unless/until she's ready--and be prepared for the possibility that it might never happen. In the meantime, you can bring the world to your MS friend when you visit: food, card/board/video games, conversation, fun, humor. Accept her the way she is--but also gently encourage her from time to time to venture out with you and emphasize that you will make sure she's safe.
It's wonderful to know that someone with MS has a caring friend like you! --Best, Kim, moderator