Well, if they want a federal document that makes sense, and yeah I don't think the fed issues that, we're too stratified so there's no such thing as a federal marriage license afaik.
I think the real issue is that what they want is a document that the person is not married. A state document might work if states required and tracked documentation for out-of-state marriages of residents, and could confirm that the person was a resident who would be subject to that requirement and was not, per records, married, whereas what the documents offer is documentation that an event did not occur within the state, not the absence of the condition of being married.
