"Spanish AESA has said, that his OML would be lifted if he can prove them that another pilot, in any country at all, with a similar condition, has had the OML erased from his license."
That sounds absolutely absurd, an OML is applied based on a statistical risk of incapacitation while performing commercial ops. Whether or not an OML can be lifted or not should always come down to a specialist clinician's appraisal of the pilot's current health, past history and the guidelines laid down by EASA Part-Med. Not what another member state are doing!