It seems to me that even if you had a conviction on your record for using a fake ID, as long as the charge wasn't a felony or a charge with the potential of more than 1 year in prison then its not a crime that would make you a "prohibited person" and as such not germane to buying a firearm and won't show up on the NICS.
last time I checked, misdemeanors that dont carry an adjudication that would count as 'mental status change' or a good bit of time in jail do not affect NCIS, and ones that would land you in jail only count for up to two years
Lets see where it appears he is saying it was a felony:
I know felony... convictions aren't ...
Making a fake ID is a felony many places. A fake driver's license is a felony some places.
It can count as various forms of fraud. Depending on how it is used could add additional charges.
So I gather he was charged with a felony for which he did commit while he was a legal adult over the age of 18 and managed to get out of the long term consequences at the state level. He was in a state that has the option to "expunge" it and not just "seal" his record. Meaning they act like it never happened, which if it never happened makes him legal at the state and federal level. If he merely paid his debt and made amends, too bad, still permently barred federaly.
(Ironic, if everyone including the state acts like the wrong was never legaly commited it is all better, but if the state acts like it did happen, but he is being given a second chance it is still a permanent federal ban. I guess the moral of that story is wrongs are okay legaly if nobody must take responsibility for having commited them in the future.)
He now wants to know if he will get into trouble at the federal level even though he is okay at the state level.
I would say things should work out just fine. There is always the chance you were added to the NICS and then not removed, which could create a lot of difficulties. However legaly you should be commiting no crime since legaly you never commited a crime so nothing should be lost by trying. Most likely it will be a non issue and you won't have any trouble.
If you are on the bad list it could be a royal pain to correct it.