If you found out your Leader or Immediate Supervisor was doing something against the law, like fraud, what would you do?

Report it.

Please refer to the legal term “accessory after the fact”.

When I was deployed in 1992 and our "Voting Representative Officer" who was supposed to mail off the voting slips for our sailors to vote in both the Primaries and Presidential Elections failed to do his job correctly for the absentee ballots. We found them in his berthing space at the time because he was able to hold them as a Security Officer and have single occupancy in his BOQ space aboard ship. He was never court-martialed or punished for his dereliction of duty. Instead, he was able to end his career with a General Discharge and Resign his commission.

Do the right thing but tread lightly make sure you have proper documentation ans evidence to prove your findings speak to the legal office make sure you speak to a civilian lawyer to protect your self against reprisals

Report facts to IG and then next higher supervisor or next higher chain-of-command.

Burn them like a pan of Jiffy Pop.

I had a similar situation in the mid-1980s. We were converting from one computer system to another when I discovered the possibility of fraud; it was possible to change the data on checks after the print file was created and before it was actually printed. I told my supervisor about it and he put me in touch with someone who could prevent that from happening.

Personally I was scammed by a LTC at the Pentagon when I requested payment for my leave time that I earned while deployed to Afghanistan. 10 years later and I'm still owed my money for leave time I earned and still not paid. The good ol boys at the Pentagon protected each other by not paying me what I earned.

It all comes down to the 7 Army Values that I still use today.

Loyalty
Duty
Respect
Selfless Service
Honor
Integrity
Personal Courage.

I use it at home and at work with which I am LT. for state corrections.

Fraud is rampant in or out of the military. Robo-calls, telemarketers and the like. With first hand knowledge if I had strong evidence, I would confront the individual and reason with him/her to stop. If this didn't work then I would take steps to bring attention going up the chain. Someone is bound to listen. As to using the Whistle Blower rule in effect. Yes I would turn the individual(s) in.