I'm a little confused if the distribution is still in his individual account didn't he get the gain/(loss) by the funds remaining in his account?
Or were the funds liquidated but he never got the check which may or may not have been sent to him?
If it's the former then I don't see that he has much of a claim, if it's the later the fund house is likely to simply stop payment on the original check and reissue likely without earnings.
Both the AA and the BPD comprise a plan sponsor's plan document. Therefore, to the extent a provision is delineated in the BPD without any corresponding AA selection, the BPD governs and should be followed. Not everything can/will/need to be outlined/selected in the AA and anything that is not expressly provided in the AA via a permissible selection is subject to any BPD mandates.