Not quite sure how to do this

This is just a “general” question – looking for some ideas here. I’m wondering if anyone out there has ever created a “private” membership site. I know you can use plugins and/or creative scripting to turn WordPress into a membership site. But my question isn’t so much on the “how to do this,”, it’s on “what’s best to use?”

I have someone who would like to use WordPress to turn it into a recipe/health website. She’s a dietician, so she would also like to offer ser services privately with a paid membership site. This part, no problem. But what’s sticking me is this: she wants the registered members to use their own WordPress area use as a “food diary” – where they would log into their account, write a diary post that is everything they ate that day. in their own private area, she would access their diary so she can offer suggestions, and upload files that only *that* individual can have access to.

Are there any WordPress plugins that do this type of thing? Or would I be better off using BuddyPress for a system like this? I was thinking maybe BuddyPress would be “too much” for this type of app – I don’t think she’s looking to create a social network, but maybe.

Anyway, has anyone heard of anything like this being done? I’d love to know what type of plugins I should be looking for. Thank you!