The guy probably has F2 hotkeyed on all his clients to a macro that says "make Billybob my focus target." Likewise, he probably has a different macro linked to F3 that says "make Peggysue my focus target." I think you're getting confused in thinking that just a regular F2 is being sent, which would normally highlight the second party member on that client's list.

The focus based system (which I don't use) has hotkeys that tell everyone in your party who the focus target should be. Then every spell is based on casting at the focus or assisting the focus.