Texting ex-iPhone owner is failing

Question

Hi everyone,
I know there's a number of threads around the issue where someone who once owned an iMessage-capable device, can no longer receive texts from Apple users, but I seem to have a slightly different issue.
My GF had an iPhone 4S, as did I. I upgraded to a 5s, and she moved over to a Nokia 925. All was well, until I tried testing her. This didn't work, obviously because she was no longer able to get iMessages. However, the Send as SMS option never automatically kicked in (never seems to) and the 'Send' button remained blue. My phone thought she was still an iMessage user.
So, after reading helpful replies on here, we have:
1. On her old phone, turned off iMessage
2. Gone to iTunes, and removed her number
3. Gone to the online Apple profile to delete her device - but it was not there anyway.  (my new 5s doesn't appear on my account, either)
4. I have deleted her contact from my phone, and recreated her.
5. I have tried turning off iMessage complately, on my phone
And many other tricks.
But, I am the *only* iPhone user who cannot send her an SMS. At first, no one could, and we understood it could take up to 48 hours to replicate the changes across Apple, but a week later, and while other iPhone users can now text her, I still cannot. The SMS progress bar progresses across the screen until about 95% and sits there, and then fails. Every time.
Her phone is recieving texts from other Apple users and non-Apple users just fine, and I can text and iMessage everyone else, as I always good - except her phone.
Any ideas what may be the issue here?
Many thanks,

Answer 1

I forgot to add, she has an iPad too - I can iMessage her just fine on that, by using her email address.
Still unable to send texts to her iPhone though.