This is football not handball. No hands should be allowed in the game, unintentional or not.
If this goal is allowed then we have several other goals that should be allowed aswell, remember Xavier's hand in EURO 2000.
If the hand is in natural position it doesn't matter if the ball touches the arm. But if the ball touches the arm so that it affects the play game should be stopped. If it was intentional he should be warned.