[petermo]

The definitions say a ball is deemed lost if :

a. it is not found or identified as his by the player within 5 minutes after the players side or his or their caddies have begun their search for it; or
b. the player has made a stroke at a substituted ball; or
c. the player has made a stroke at a provisional ball from a place where the original ball is likely to be or from a point nearer the hole than that place.

Any outsider who says "this is your ball' cannot force the player to identify it - he and his caddie are the only people allowed to search for it and if they decide to abandon it by not searching for it or identifying it that's their prerogative.