My iPhone 4S won't text my friend's Android, my phone thinks she still has an iphone with iOS5

My friend used to have an iPhone 3GS with iOS version 5. Our text messages showed up blue being that we were on the same software. She now has updated her phone to a Galaxy S II and my phone still thinks that her phone number is operating on iOS 5 software. Her phone cannot receive messages from my phone, but my phone receives messages with no problem. No matter what I do (even restarting my phone and deleting her and reading her) her text messages are still in blue instead of green. When I send a message, it shows that it's sent but her phone never receives it. It's extremely aggravating.

This is a known issue with iMessage. In your case, what has happened is that the SIM was registered on Apple's servers as capable of accepting iMessages. When your friend swapped phones, they likely kept the same SIM card. So now their old iPhone (bound to Apple's servers by its UUID) and the SIM card (bound by its own ID) are both registered with Apple to receive iMessages.

This article talks of possible fixes, and the possibility that Apple has something cooking (after all they should provide some capacity to remove the devices, perhaps from your Apple ID account). You can also head over to this article for additional reading and perhaps more solutions.

I wish I had a simple solution for you, but it may require calling Apple and discussing the matter with them. I would say you should also ask them to cover the cost of a new SIM should your friend require one.

Well it is a bit of a heavy handed solution, but if you were to go into Settings -> Messages and set iMessage to off, then all of your outbound messages would be sent as regular old SMS. Your friend would get them then.