AIBU regarding Social Networks

[confuseddaddotcom1]

Hi, I would like some advice about my stepdaughter. She is 15 and has some degree of Aspergers. She is very intelligent but over the past 12 months has been talking to strangers on social media, she also believes to be romantically involved with a girl in America and has in the past been sharing quite explicit pictures with her and discussing sexual content with her. We have banned her from internet access/phone apps several times but has returned to talking to this individual. Recently there has been no evidence of this relationship on her phone but we are unsure if she has become wise and started deleting the conversations. Her most recent ban was down to attitude and being quite agressive/confrontational. We are considering giving her access back but to safeguard her, advised her mother that having full access to her logins/accounts on her mothers phone would be the best way to deal with potential issues with conversations. Her daughter became agressive and severely upset with this as she felt this was 'stalking' and did not want us to have access. I feel spot checks could be manipulated and the conversations deleted immediately afterwards which is why if we had full access to her account would be better. Am I being too invasive or is this a good way to deal with the situation?