This may change the situation a bit - I'm not aware that A3 lounge staff wander around checking eligibility to be there (at least, I've never seen it done), but technically I believe your guest should leave the lounge no later than the time you do. Overstays of a few minutes or even an hour are unlikely to be detected, but over that period - given how small and limited the lounge is - you are going to be remembered. However unlikely, it would be extremely embarrassing for both of you if your colleague was asked to leave - plus, honestly, he would be driven absolutely insane beyond about the three hour mark and would want to be out wandering the terminal shops at the very least, which would prevent re-entry without you.