As per Ballotpedia:
Primary: Idaho is one of 22 states with a mixed primary system. Parties decide who may vote. The Democratic Party allows unaffiliated voters to vote in their primary. The Republican Party allows only voters registered with their party. Unaffiliated voters can choose to affiliate with a party on election day, but they will be bound to that party at the next election as well.
Voter registration: Voters had to register to vote in the primary by either April 25, 2014
He could take a dem or unaffilated ballot because the dems have an open primary. The Republicans do not. The deadline to enroll was April 25.
As per KTVB.com:
Ada County Clerk Christopher Rich said the new process of the Republican closed primary has been confusing for voters and poll workers. This is the second election using the process.