Yup - I agree with soup, no doubt that there will be another account set up that your DP knows nothing about.
But that doesn't actually matter. Because your DSD knows that he stepped in, stood up to her mum when something was important enough to him in order to try and keep her safe and she will remember that.
We went through all this drama - every time DH found out his underage DC's had a social media account, he reported it and it was deleted. His ex used to go beserk, his DC's would be pissed off, but then listen to him when he explained reasonably why he was concerned.
So when DH's DD got herself involved in a Twitter drama that resulted in her receiving death threats, she came and told him. Not her mum, who had let her have unrestricted access to social media, but her dad, who had always been concerned about her online safety. Together they sorted it out, involved the police and eventually, the perpetrators were prosecuted for making death threats to female politicians.
So, even if your DP's actions don't stop this in the short term, the messages he is sending his DD may well keep her safe as she gets older.