So my understanding (and correct me if I'm wrong here) is that when I guest someone into the lounge the staff at the door scan both boarding passes and 'tag' them together, in order to prevent me going in and out with different guests to circumvent the 1 guest rule.

But in theory this should mean that the system knows that my guest has been guested in, and so can access the lounge on that boarding pass, and so in theory they should be able to leave the lounge and then re-enter without me.

So the question is: does it work like this, or is BA's famous IT too inept to know that a guest has been granted access already?

I ask as I've only ever guested people travelling on the same flight as myself, so we always entered and left the lounge together. But next week I'll be flying on the same day as a friend who has a flight which leaves about 90 mins after mine, so I told him that when he arrives I'll pop out of the lounge and bring him in (I won't have guested anyone else in beforehand), but having done so would he be able to leave the lounge and re-enter after I've left for my flight, or will it be a case of no re-entry for him?