Well like you said she is needy. I’m thinking she doesn’t have any other friends either, maybe some acquaintances but not real friends. Because of this she is turning to you every time she needs to talk to a friend.
I don't know that this part is right, but you nailed the rest of it. (RandomDrew/last post on the first page) I am thinking that the "married" friend is waiting for her to come home from one of her three jobs, and since the OP is working all of the time, that is the only time the old friend has with her. I would think that they have been friends for a long time. I am guessing, but they probably spent much of their teen lives together, adn have had a very close friendship. The OP deserves her time to date and her own space, but her old friend used to be able to spend a lot more time with her. Your suggestions for gaining and holding her own space are very helpful. The boundaries were not set in the teen years.
Good luck, Alicia.