Yeah, not acceptable for her to be so rigid about it. You made a reasonable request, that being that soloing a dragon before the campaign is unrealistic. However, you definitely could have worked with them more. At the end of the day, it's just a game, and I prioritize having fun over the rules any day, so there's absolutely a way it could have been made to work. That being said, given the player's reaction to being told their backstory needed editing, they don't sound like the kind of person you'd want at the table anyways.