Authorship Guideline for unused experimental results
I think in a larger sense your intern certainly did contribute to the project... whether or not the specifics reflect that. They were part of the team. Many things in team work are unpredictable... but everyone on the team should get credit. Otherwise, think of it, people will not agree to participate and exert effort in situations where they're not guaranteed a good PR outcome. That attitude would wreak havoc with genuine scientific/intellectual inquiry.