If he's serious about aviation he needs a good lawyer. If he can beat this, or get it reduced to something innocuous then it should have minimal effect on his career.
If he get's convicted the long-term impact depends on the details. If this is vehicular-related but non-DUI it might be no worse than reckless driving...he'll still get hired at regionals but majors will take AT LEAST 10-15 years with a clean record and even then only if they need a lot of pilots. When airlines can be picky, there's always plenty of pilots with clean records.
If it's a DUI, that's worse and I would seriously question investing in aviation unless it can be beat (ie charges dropped or not guilty). A DUI plead down to a lesser charge is still likely to be a problem in aviation (you have to tell the FAA what the arrest was for).
If it's non-vehicular but related to violence he would need to beat the rap. Any conviction related to violence is probably game over. Yes, employers know your ex-wife probably fabricated the whole incident to get custody of the kids but they don't know that for sure and have no way of knowing so they'll just hire somebody with a clean record. Airlines are not in business to be fair to all applicants.
Hard to answer precisely without more details. Regionals will hire almost anyone but majors are very sensitive about what google will turn up about one of it's pilots.