The DMV isn’t a court. They can hold hearings and suspend or revoke a drivers license, but they can’t convict you of a felony, that what an actual court does…think Dept of Justice, not Dept of Motor Vehicles. Reference 46 CRF Part 10:
Conviction means that the applicant for a merchant mariner credential has been found guilty, by judgment or plea by a court of record of the United States, the District of Columbia, any State, territory, or possession of the United States, a foreign country, or any military court, of a criminal felony or misdemeanor or of an offense described in section 205 of the National Driver Register Act of 1982, as amended (49 U.S.C. 30304)…
There are a lot of threads discussing similar topics on this forum, your friend might want to search/review those for anecdotes from people who have gone through similar.
Also consider the second part of the Definition of Conviction especially as it relates to this part of your comment:
That’s probably fine as it relates to his drivers license, but if he takes classes etc as part of getting out of the DUI, that will likely still count as a “conviction”:
If an applicant pleads guilty or no contest, is granted deferred adjudication, or is required by the court to attend classes, make contributions of time or money, receive treatment, submit to any manner of probation or supervision, or forgo appeal of a trial court’s conviction, then the Coast Guard will consider the applicant to have received a conviction.