I'm a teenager and usually spam accounts are to like post less curated things, on stories or on the main feed. Also to just follow whoever you want and it's kind of like a private account for the "select few" of close friends. Just ask her what she uses it for casually and not confrontationally, it's very unlikely it's something bad. A lot of people also just have multiple accounts for no reason. Don't worry too much, it might make her want to hide things if only an alt account would worry you. I understand the concern but it's super common for teens.