I've never been turned away from a premium check-in counter by AA or any other airline when checking in with a non-status travel companion. I don't think they'd turn your friend away just because he's going to a different destination, but I don't make their rules. (Too bad. I'd change some if I did.)
I checked in at the LHR Flagship area around 9 am this past Wednesday (Feb. 23) for AA 109, 10:30 am LHR-BOS. I was the only passenger in the place. There were one greeter, two security checkers and three agents there to serve me, plus perhaps others behind the scenes whom I didn't see. If it's that quiet when you're there, I suspect they'd be grateful for something to do!