Why did he do two challenges? Did he not meet the requirements of the first one, or wanted to go for a higher status the second time?
As for "What if they only found out 1+ years later and you have earned enough miles to qualify the next year (the normal way)?", well:
https://www.delta.com/content/www/en...ram-rules.html
Delta reserves the right to deactivate or close an account under the following circumstances:
Fraudulent activity occurs.
[...]
It's the first thing they mention
So, good luck with that.