Congrats to your client.  Not easy to get into an airline cadet program.  @BobKamman  said "What if he decides to work for another airline ..."    Short answer is the contract the cadet signed.  Cadet programs generally come with a 2 year employment commitment IF the cadet successfully completes the program. and is offered a job. (fwiw, they're entry level regional flying jobs at relatively low pay usually requiring relocation... you don't get to pick your duty station).

Cadet programs have been around for a few of decades. Acceptance into the program is competitive, requires testing and several rounds of interview as well as minimum pilot ratings, hours and, of course, a thorough background check.... 

When an airline does accept a cadet into the program, they have a pretty solid idea about the cadets commitment, determination, and motivation.  The programs have a very high success rate. Some of the money the programs pay the cadet is defray  a portion of the advanced pilot certifications and ratings. An IFR rating can cost >8K.  and ATP rating >5K. ..  

As for the question of 1099-Misc or 1099-NEC,  I'd  defer to the airline legal departments to have researched the appropriate classification. Yes, I'd put it on a schedule C subject to SE. There may be some reasonable expenses incurred but you'd have to read the Cadet contract. The cost of ratings or other training is not deductible.