Asking for a friend who: is British, born, raised & living in UK, with UK passport; has a Spanish father; did not obtain Spanish nationality (before the age-limit of 18/20); is now wondering about acquiring Spanish nationality, for which he would only need 1 year of residency in Spain, since he has a Spanish parent.
Question: If he moves there for a year or two, to achieve this (with the necessary registration/NIE/padron), how much is he able to move between Spain and other countries (mainly UK and maybe France) during that time, without jeopardising the eventual nationality-application?
i.e. Will they eventually check how many days he spent outside Spain? Will they check for entry/exit stamps (which are not usually given between these countries) During his residence, will the spanish authorities do spot checks on a foreigner's registeered address? What if they were away at the time? (whether that's touring round Spain or abroad)?