We think her concern/excuse is 'legitimate'. = We think she has a 'legitimate' concern/excuse.
It's not clear that the letter is 'legitimate' [='genuine']; it may be a forgery.
He can 'legitimately' [='rightfully'] claim to be the best athlete in his class.
(..) You know, I really think that would be good for you. A clean slate. All right. Just think about it. You get a job. Something legitimate. Something you like. Meet a girl. Start a family, even. Hell, you're still so damn young. And what's here (..)2013 TvShow: Breaking Bad Title: Confessions Season: 5 Episode 11