I'm in the US. One thing to remember is that each state makes its own laws regarding MH issues. Where I live APS (Adult Protective Services) are 'devolved' to each county by the state. They work within a legal framework, but things can vary a bit from country to county. Other US states may handle things differently
Basically (again, where I live) APS steps in for (quoting) "elder adults (60 years and older) and dependent adults (18-59 who are disabled), when these adults are unable to meet their own needs, or are victims of abuse, neglect or exploitation".
As it stands APS wouldn't step in because Britney has not been determined to be mentally incapable, nor (as far as we know) is she a victim of abuse, neglect, nor exploitation. I'm not being 'snotty' or uncaring here, but a person is entitled to post nude pics or make rubbish statements of their own volition if they want to, even if it doesn't appear to be a very wise thing to do. Doing so doesn't make one automatically 'non compos mentis'. Many people post nude pics of themselves. I wouldn't do it, most of you wouldn't do it, but doing it doesn't make someone incapable of making their own day to day decisions.
If someone in Britney's life feels she's being exploited or coerced to post these pics or if they feel that her judgement is so impaired that she is not capable of understanding the ramifications of posting such pics, they can report her to APS.
Do I think Britney has MH issues. Yes. But I have no way of knowing what, if any, help she is getting. All I can do is wish her well and hope that she has some people in her life that have her best interests at heart.