I'd do something like this to find the right player, not sure how to make him drop it.
Code:
public getBombPlayer() {
new Players[32]
new playerCount, i
get_players(Players, playerCount, "gh")
for (i=0; i < playerCount; i++) {
if(user_has_weapon(Players[i], "weapon_c4"))
return Players[i]
}
}
(Not tested)