There is no joint and survivor annuity for non-spouses under the plan. There is only an SLA, and 5-year certain option on the SLA.
I hadn't thought about the Code defining a QJSA for a non-married participant as a SLA - that is good point. If he had been paid a SLA, he would have had a bigger monthly payment.
It seems to me that the spouse would have to have been a spouse on the ASD, whether because an actual spouse or pursuant to a QDRO. The participant did not remarry after the divorce, so there is no other potential spouse.
One concern I have is that there is a QDRO or at least a divorce order touching upon the benefit and no one provided it to the plan. Perhaps that is why the participant listed the ex as his current spouse, thinking that was the way to handle it.