IOS – Does the iPhone 4 still vibrate in your ear if you receive a text message during a call

Either the software isn't seeing the proximity sensor or your hardware needs a repair.

I have had good luck testing the phone immediately after a normal reboot (slide to power off and letting it shut down) Then place the phone flat on the table, press sleep/wake to power it on and once it starts - just call 611 or another free call before starting any apps. Place your whole hand over the top half of the glass and play with the sensor that is near the receiver / front camera to see that the screen goes on and off as designed.

Restoring the phone (and sometimes setting up as new) is needed to reset the software when it fails. Calling or going to an apple store for assistance or repair is the next step once you've decided it's not fixable from simple software.

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.