From
http://www.britishairways.com/travel...e/public/en_gb
Lounge access is subject to capacity restrictions and guests must be travelling on the same carrier as the Executive Club member
And if you're talking about 'Open Doors' for Golds when you're not on a OW flight, you don't get a guest anyway with that.
Strictly speaking, I believe your guest
should be on the same flight as you - they want to discourage you coming in with a guest on a flight 8hrs later and having them over-indulge once you leave, I think! I have got in guests on different flights before but it's always been BA flights and generally leaving quite close to mine.
Phil