Yes and no.
The Rules of Golf do not specifically require you to help your opponent or fellow-competitor search for a ball; or to tell that golfer where his ball is if you find it. So the answer, according to the Rules, is a qualified "no."
However, sportsmanship, golf etiquette and the spirit of the game all say yes - emphatically, yes. Our own personal answer to the question is that you absolutely should tell your opponent that you've found his ball.
The Rules of Golf do address this indirectly in its preamble on etiquette, under the headline, "The Spirit of the Game":
Unlike many sports, golf is played, for the most part, without the supervision of a referee or umpire. The game relies on the integrity of the individual to show consideration for other players and to abide by the Rules. All players should conduct themselves in a disciplined manner, demonstrating courtesy and sportsmanship at all times, irrespective of how competitive they maybe. This is the spirit of the game of golf. (Bold facing is ours.)
When the Rules of Golf were revised for 2004-2005, the USGA and R&A gave more authority to the Committee to punish a "serious breach of etiquette." Rule 33-7 states:
If a Committee considers that a player is guilty of a serious breach of etiquette, it may impose a penalty of disqualification under this Rule.
So while the Rules of Golf do not specifically require a golfer to inform an opponent that he's found that opponent's lost ball, the Committee could, in theory, rule such an action (or inaction) as a "serious breach of etiquette," with disqualification resulting.
It's probably not likely that that would happen. But regardless, withholding such information from an opponent in hopes that the opponent will have to take a penalty is a breach of the ethics of the game.
So if you're ever faced with that decision, no matter how you feel about the opponent or fellow-competitor personally, or how competitive you are, do the right thing. Bobby Jones is whispering it in your ear: Show yourself a true sportsman and uphold the spirit of the game.
