That is actually a really, really common thing for over-controlling parents to say. The kid was perfect until they got old enough to start disobeying the parent, or just making decisions the parent didn't understand or pre-approve. At that point, the parent blames this unsettling problem on the kid's friends, who have been a bad influence on the kid. Or in more extreme cases: "brainwashed" the kid, "turned them against me," or "just walked in and taken over their whole life!"

(Though if there's a custody split, the other parent/guardian may get assigned the job of Corrupter instead or in addition. Other sinister explanations for children having their own personalities: teachers, the internet, books/TV/games/etc, or psych/health problems (real or imagined).)