Is this UA420? I initially assumed they'd swapped a 739 to a 738 which removes 4 F seats and does happen sometimes, but that flight shows as a 739, so only reason someone would get bumped is FAM (UA essentially never overbooks premium cabin prior to departure).
This is actually worse than your subject line suggests -- it's both a loss of seat assignment *and* being bumped for the flight. A couple options:
1) If you can check him in, presumably with a seat assignment of "See Agent", he can go to the gate and possibly get the seat of a no-show. Worst case, they'd have to downgrade him and he'd receive substantial compensation (GG OVS Downgrade will get you the relevant thread, though it's not clear that FAM count as "oversale" for these purposes).
2) Otherwise, I'd accept the agent's offer to get reaccommodated on an earlier/later flight, then send UA a simple message tomorrow stating the facts -- ie with no prior warning, bumped from flight, requiring a phone call to an agent to fix. I'd be shocked if you didn't get something even if it's FAM-caused.