My middle DS was terrified of flying when he was that age. We had a trip planned to fly to California, and we were so concerned about him possiby freaking out at the airport that we talked to the pediatrician and even called the airline to see what they would do.
This was all made worse the anniversary of 9/11. He came home from school talking about planes crashing into buildings (they'd done a remembrance thing in kindergarten). He was afraid before that, but of course, this new information made it way worse for him.
The pediatrician told us to talk to him before, to explain that planes were safe and why. He said that if we showed any hesitation or fear or nervousness ourselves, he would pick up on that and get worse. United Airlines told us that if he screamed and did not want to get on the plane, they could refuse to let us on.
Anyway, we did talk about flying before, and we played up the fun stuff--he'd get to eat a snack, listen to music, maybe watch a movie. We let him bring some toys of his choosing, and we also packed a couple of new toys, too. He ultimately did OK, but even now (he's 8) he does not like to fly, and we give him Dramamine because he gets sick.
I would advise your friend to do what we did--and that is to talk about what will happen on the flight and how safe it is, and how the pilots go to school for a long time to learn how to fly. I really think our pediatrician was right when he told us that our kids pick up on our moods. If we're OK with flying and even excited about the trip, the child will do better.