It all depends on WHEN he was told. IANAL, but it is my understanding that contractual obligations can't be added after a purchase. Only before.
The dispute will probably hinge on the fact that the app was not bundled with the headphones.
He'll probably argue that he purchased the headphones based on the availability of the app, and the headphone case didn't say "conditions apply" or something.
If the app was free, he's probably SOL.