I feel sorry for her, although she's definitely taken it too far. She's probably just so desperate for a friend that she can't help herself, iykwim. I can probably give her POV because she's me to some extent.
I guess I'm the in the same boat, in that I don't have any true friends. Unless I text the few ppl that I do know I don't hear from them and I really don't think they'd even notice if I dropped dead; the only difference is I don't constantly text/call etc. I'll wait and see if they contact me and after a few months when they haven't I'll text them and we'll usually meet up for a couple of hours or less for a cuppa and that will be that till the next time I cave in and text.
I think I'm a nice person with a sense of humour as well as a loyal friend; I do volunteer work and I always try to help others out; I was always the one helping out if they were stuck in a jam.
I suffer from low self esteem (obviously; how could I not when no one seems to really like me? ) and I get very upset that I don't have any friends.
I'm ok at speaking to ppl but it never gets past the acquaintance phase (no coffees, etc) so I can totally see how she had become so clingy and needy.
The problem is; every time someone rejects you it exacerbates the problem and you cling more which is the wrong thing to do. She's probably really lonely and the harder she tries the worse she gets. I'm at the stage where I'm kinda resigned to never having ppl care about me and want to be friends (not that I was ever that bad).
My biggest thing was worrying that I'd offended them when they didn't ever reply, which would make me paranoid about how I appear/behave in public.
I once asked a longterm friend (35+ years) about it (I was first on the list if she needed a favour/babysitting etc but not for anything else) and it turned out I was too fat didn't fit in with the group to see more of her (I was often the target of her 'teasing' when we were in a group). At the time I put up with it as I was so desperate for company; but I don't anymore even though I still don't have friends.
I guess I try too hard sometimes, but it's hard to be normal if this is your life. I was severely bullied throughout my school years so I struggle to make friends anyway.
You definitely need to nip this in the bud, but, and I'm sure you will as you come across as really nice to have put up with it so long, try to be gentle about it and explain to her exactly why things need to cool down drastically. I think that was always the most upsetting thing: I've never known why ppl treat me this way so I can't change it or work on it.
Good luck. 