It sounds like the dysthymia has evolved into real depression. Your friend might also have a social phobia problem.

In any case, I think the best thing you can do is to invite her to go out of the house with you from time to time. I think the sooner you start, the easier it will be to get her moving. Just plan little short trips. Ask her to go to the store with you. (She must need to do some shopping occasionally, herself.) You can also visit with her in her home, but I would not do too much of that, or you could be reinforcing her staying stuck in her place.

It's good of you to care about her. She is very lucky to know someone who does.