Sharing the Kicker..problems

Last week i watched a friend playing 4-8 for the first time. One hand he held Q-J . At the turn they played head up. The board was J-2-7-2. There was a bet and a call. River was an Ace and both checked. The other player showed J-K and my friend didn't hesitate to throw his cards in the muck. He didn't see that the Ace on the river delivered him a tie-hand because it became the very best kicker for both players. Allways look out for situations like that when the board is paired. A mistake can be pretty expensive.