Access to private member from static helper function

I'm looking for a way to implement friend/private functions that requires access to a nested private class. I'd like to avoid declaring it in the header file, to abstract the implementation details and to avoid bloating the header. These functions should not be called outside the source file they're implemented in as well.

Ideally it would be a static function, but somehow with access to private class/member.

I've read that a nameless namespace would help me solving the latter issue at least, is this correct?

Thank you.