There a setting in Google Talk that makes you "Unavailable" when screen is off.
Tap the three dots, top right, select settings and Google account...
The setting is under General and "Away when screen is off" will be ticked, I would guess your friend with the S3 does not have this ticked?
Hey,
Not sure what you mean by the "Unavailable" option, but we both do have the "Away when screen off" selected on our devices. Thanks