The "capacity" "loophole" would be more convincing if the lounge was actually busy. But it wasn't, no doubt a combination of the time of day, day of week, and season. This rationale for non-admission would also be more convincing if the attendant had cited that exception, but she did not. Rather, the lounge attendant indicated that IB passengers are never admitted at all, at any time. If BA wanted to avail itself of that exception, it should have mentioned it.
If someone took this sort of case I'd be rooting for the defendant and hoping costs were awarded against the claimant.
Sounds like you're talking about the British system. This is Boston, USA. We generally don't award costs to a prevailing defendant -- only in exceptional circumstances when a statute so provides.
if ... your friend ... doesn't get satisfaction then publicise issue or complain to local equivalent of trading standards / advertising standards authority.
The applicable authority in the US is the Department of Transportation. DOT, and not FTC and state regulators, is empowered to investigate all manner of "unfair" and "deceptive" practices in aviation. Denial of a promised passenger benefit is properly within the DOT's mandate. I agree that my friend could also consider a DOT complaint. On one hand I don't know of DOT doing much as to passengers' lounge access -- but then again the complete unqualified denial of a benefit, contrary to written materials, is an unusual fact pattern. Most airlines are not so brazen.
what relationship does your friend have with BA? Your friend is an AA EXP, and flying IB. There is no contract implicit or otherwise between your friend and BA regardless of BA's participation in the oneworld alliance.
I agree that access as EXP makes things more complicated, raising a question of who the passenger should look do when the loyalty program, operating carrier, and lounge carrier are three different companies. But recall the quote above:
If your oneworld frequent flyer tier status is Emerald or Sapphire, or you are travelling in a First or Business Class cabin on any oneworld member airline, you can access some 550 airport lounges throughout the world.
My friend would surely rely on the bold language, for entrance based on class of service. Here there are only two carriers, BA and IB. Meanwhile they're under common ownership (both owned by IAG).
BA is not a party to the contract.
"The contract"? There are multiple contracts at issue. There's a contract between passenger and IB for transport. But then there's BA's unilateral offer quoted above -- buy a business class ticket and access our lounge. That's a unilateral contract which a customer accepts by buying the required ticket and presenting him or herself at the specified lounge for admission. If you're not familiar with the concept of a unilateral contract, you might read
this definition. (In short: A contract in which only one party makes an express promise, or undertakes a performance without first securing a reciprocal agreement from the other party.)
I wouldn't waste any more time or effort on it. If you attempt to take this to a court you will fail and end up very much out of pocket.
Are you familiar with the US small claims system? A consumer can file a small claim at minimal cost (~$30). We're not taking about thousands of dollars of attorney time (which, I agree, would be harder to justify). This can be an effective way to alert a company -- its lawyers and its decision-makers, not its customer relations staff -- to a breach of their legal obligations. It should not be dismissed so quickly.
BA here announced a policy and benefit, and through its close partner (under common ownership) sold a $6k+ ticket. It then disavowed the policy and benefit to its own advantage and to the customer's detriment. I credit that most people would shrug and move on. But the passenger has a valid complaint.