I want to get a friend to buy me a spark we he's abroad
The battery needs the be discharged to 5% for taking on an airplane
Can this be done using the app and how?

I'm not sure on the spark, but on my Phantom you connect the drone to the app, go to settings, there is a tab for the battery, and in there you can set how long until it goes into storage mode. Change it to 1 day. Or, fly it to 5% (which will be faster and more fun)