If he must do it on an airplane, make sure he springs for first class and, if possible, select some seats that would give them the most privacy. As others have said, please strongly discourage him from trying to do it on the PA. It's extremely tacky, annoying to other passengers (whom may be sleeping) and could be very uncomfortable for his girlfriend.

If she is truly that big of a plane fanatic, this could be romantic if they were in adjoining first class suites on EK, or something similar, jetting off to a foreign destination. However if he's thinking first class on the 4:00pm DL ATL to JFK in an A320 is a appropriate setting, he might as well do it on a city bus where he could at least make a quick exit if she says no.

Just my two cents...and you know what they say about opinions. Still, good luck to him!