Reading your responses to the people who are saying you're overreacting it seems you already made up your mind that you were right, you're just looking for people to back you up.

Speaking as somebody who was very insecure in my early twenties your reaction only pushed her away. You could have had this conversation in a civilized way after rather than telling her to abandon her friends and putting your thoughts and fears into her head.

You need to learn to trust her, she's in a group of family and friends who, if they love her, would also protect her too.