My thought is that, as Mr Spock once said on Star Trek, a difference which makes no difference is no difference. I am happy to be argued out of this position – it isn’t something I feel strongly about. Another example might be someone claiming to be gay but continuing to live exclusively and happily in a hetero relationship for their whole life. It could be said that labels are for tins of beans, not for people, so does it matter at all? I’m not convinced either way on this.
Nobody has called my friend a cisgender man, so far as I know. My argument is that he/she acts as one in every part of his/her life.