Even without a conviction, a record will exist of the arrest as entered into the national crime information computer, and will come up during a background check.
I know an individual flying international 121 that shot someone. I know others with various convictions. These are not necessarily show stoppers, but one would be very remiss not addressing the matter with legal counsel.
If your friend is involved with AOPA, a legal plan exists for a very low amount which provides some relief in locating and obtaining counsel no such matters. Fifty bucks or so to joint, then about a hundred bucks a year for the professional level of the legal plan. Very well worth it, especially after AOPA increased the scope of the coverage several years ago.