In spite of the district court's merciful sentence, he has committed an aggravated felony in the eyes of immigration law and so is barred from ever entering the US again. For life.
IANAL but it sounds like he may be able to appeal based on a recent SCOTUS ruling[0].
> The result is that people convicted of certain crimes -- such as the California crime of burglary -- that are not by definition necessarily violent, may not be deportable.
[0] - https://www.shouselaw.com/immigration/aggravated-felonies
For entry into the US on visa-waiver (ESTA) or visas (without a green card), you generally can't appeal to the courts, and court rulings about deportation aren't really relevant.
It is up to the discretion of CBP (and also the State Department for visa issuance). They can decide to disregard a criminal conviction - they are more likely to do that if it is relatively minor, if there are some unusual/special circumstances, if it is from many years ago, if a person shows evidence of being of good character since then. But it is totally up to their discretion.
If they rule against you, there is no formal right of appeal. You can talk to your own country's government, ask them to make diplomatic representations. If your own government decides to do so (they are under no obligation to do so), there is some chance they might change the US government's mind, but no guarantee.
That kind of thing could definitely be relevant for showing good character since his bygone days as a malware creator rather than researcher.
(If you can make the case that the visa/entry refusal was due to some improper reason, such as racial or religious discrimination, political vendetta, government corruption, etc., then you might have some chance, but even then the odds are not that great. But if your case is simply "they won't let me in due to my prior felony conviction in a US federal court but I don't think that's fair", then your odds of success are almost exactly zero.)
(Disclaimer: IANAL.)