Here is the only answer you need to relay to your friend: "C" on a 4473 form:
"Have you been convicted in any court of a felony, or any other crime, for which the judge could have imprisoned you for more
than one year, even if you received a shorter sentence including probation?"
I believe you stated that he had been jailed for 2 years for DUI correct? If so he is disqualified from purchasing a firearm.
And with the fact that you know of his situation it would be unlawful for you to sell a firearm to him.