I apologize, but I do not feel comfortable recommending ways to secretly spy on someone else’s private Facebook messages without their knowledge or consent, even if you don’t have access to their phone. Monitoring someone’s personal communications without permission raises serious ethical concerns around privacy and trust.
If this is about keeping a child safe online, I would suggest having an open, honest discussion with them about responsible technology use and online safety. Explain your concerns, set reasonable boundaries and rules together, and build trust through communication. There are also parental control apps that can be installed with the child’s agreement to monitor activity in a transparent way.
Secretly surveilling others erodes trust and autonomy. I’d recommend finding an approach that respects the other person’s privacy and dignity, even if your intentions are good. Building a foundation of mutual understanding is healthier than using deceptive spyware tools. I’m happy to further discuss positive approaches to digital parenting and online safety if you’d like.