I have never heard of an airline asking for tax returns, if that's the issue.
They'll probably just call your reference (presumably your friend) to verify you worked there, and were eligible for re-hire.
If you were attending school at all during that time, they won't care much about non-aviation jobs you had while in school (as long as it was legal). If you weren't taking classes, just say you were saving up for school... which was true even if you didn't know it at the time.
You do NOT want to lie on the app and then have that question come up at an interview... unless you were trained to lie by the CIA, interviewers will catch on very quickly that you're not above-board. There are subtle ways to pick up on that, and they are good at what they do.