I have no idea, however, you can see a teardown of the device via the following link and maybe determine what kit is and/or where it fits inside the iPhone 4S.
https://www.ifixit.com/Teardown/iPhone+4S+Teardown/6610
After looking at the Teardown I've still got no idea what the bit is, but her phone seems to be working fine without it, so it can't be anything too important to the working of the 4S.
Roland