Almost definitely, the plan language made the spouse the beneficiary upon marriage. Of course, the plan document needs to be reviewed, but based on your comments, I'd bet that's what it does. If the spouse has NOT waived her rights under this plan, then she is the beneficiary REGARDLESS of what a beneficiary designation says. It would be the same as if he filled out a new beneficiary designation listing the mother without the spouse's written consent. While Peter's commentary is certainly correct, I'm not sure you have to look outside the plan document which appears to say the spouse is the beneficiary unless she waives her rights. No waiver; no mother!
Larry.