16 years ago, I proposed to my wife on a plane.

At the time, she was a Flight Attendant working the flight. I was also an employee and was non-reving on the flight. We had already had discussions about getting married, she had told me her ring preferences, and I had the talk with her father (which she knew about). My dilema at the time was not whether she would say yes, but whether she would be at all surprised. The reason that I chose to propose on the plane was that it was out of character for me and she wouldn't expect it.

There were some logistics involved. I had to get to the airport separately. I didn't want the security screener to look at something oddly shaped in the carryon bag and say, "Hey, what's this?" I also wanted to talk with the Captain and the Number 1 Flight Attendant to get their permission to use the speaker. It was somewhat of a logistical challenge to talk with them before the flight left and not get my now wife suspicious. The Captain, the Flight Attendant, even the gate agent all saw the ring and were thrilled about it.

I was lucky enough to be seated in F. All the other Flight Attendants came up to see the ring. Soon the ring was being passed around the F cabin. After the service was done in the back (remember when there were meals on domestic flights?), I got up, went to the loudspeaker, and began to talk. I met her at the curtain, got down on one knee, and she said "Yes." The other passengers seemed to enjoy it (especially giving me advice).

So to the OP: Advise your friend to get there early; Talk to the pilots and FA's; and don't listen to the naysayers!!