I've been wondering about this topic for a while. It does make sence for airlines to charge a fee for lounge access. My guess is that the airline providing the lounge access (frequnet flyer program or class or service) pays for it unless operating airline provides it (customer flying in business, first or py for some case).

I've done JL C to AA domestic flight several times. Everytime agents scanned both JL boarding pass and AA boarding pass. One time, an agent wasn't sure how to "register" me (she was talking to an agent next her saying "I don't know why I get an error message. I am not sure if JL pays for it") At the end, she reprinted my boarding pass led me in to AC.

When I used CX and AA's lounge at NRT, agents at both lounge wrote down my flight number and seat and looked for ow status (which I don't have). I guess JL would get bill for both situations. Or If I had an ow status, a fee may have been split into two parts?

My question is when airline uses the third party lounge (like AY at CDG), which airline would be responsible for paying a "fee". My guess is that AY will pay first, but aren't they "charge back" that fee to carieer that provides lounge access?

I guess this kind of information is not available to publicmc, but any guess/comment is welcome!