You don't mention specifically what job your friend is looking to do with an airline. Managment? Line pilot? Flight attendant?
If she's looking to be a pilot or flight attendant, it is unrealistic to expect to take 5 years off and then jump right back into her place on the senority list. Most airlines have assorted varities of personal leave, but like Doug said getting a leave approved for just 'personal' reasons can be hard to come by. After that length of time, she'll be most decidedly non-current, and would have to undergo currency training.
Dropping/swapping/selling individual trips is quite common, at least in the flight attendant world. There are some flight attendants who, via swaps, give away or sell (as in pay another flight attendant a token price, like $50-$100) their trips and hardly fly at all. AA became irritated by this, and instituted minimum hours a F/A must fly per year to remain on full-time status and continue to be able to qualify for benefits.