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.
If your friend has registered her iPhone with Apple in the past, she can unregister it on the My Support Profile website.
This article (How to turn off iMessage without iOS device) on Apple’s forum pages implies that this device is then also removed from iMessage, the original problem being:
My iPhone got stolen, but when my friends message me their phones keep trying to send me iMessages.
Having similar issue between droid and iPhone… Cannot get each others texts. We both downloaded the app 'WhatsApp' and with a super easy set up we are now able to text without any issues. We can send photos and the app even says you can make calls with it. Its a quick, easy fix to this problem. So glad we found it!
I had come across this issue regarding the same phones. We figured out that, because the old iPhone was switched on, my iPhone picked up on iMessage, once we turned the iPhone off as It wasn't needed then the Samsung S3 recieved all msgs as a normal text.
But if you have sold your iPhone, and not deleted your iTunes account, this will not work. You will need to delete your account. If for any reason you don't wish to do this because you have other apple products it will result in registering a new SIM -- which takes five minutes once you have a new SIM.
Check your phone plan with your carrier. My plan is the same one I started with in 1994. Make sure that your carrier does not show text messaging blocked for your account. I finally went to my Verizon store and there was a guy there who recommended the person who was helping me check my account for any blocked services. Apparently my vintage flip phone did not allow texting; as soon as he unblocked my account, I could finally send a text to my droid contacts. I've been dealing with this problem since last August.