Restrict front-end categories on a per user basis

Heya,

I am trying to add the functionality to my site for a personal training business. The flow would be like this

1) The “athlete” registers (Or the Trainer provides their athlete with login details).
2) Athlete can write their own blog posts and publish.
3) The Trainer can comment on the posts of the athlete.

I will use gravity forms to make the form I need for the athletes to write directly to a post, and make the user role author so they publish straight away.

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TRICKY PART
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1) The tricky part is that the athlete should only be able to see their posts and not other athletes (privacy reasons).
2) I dont want the athlete to ever go into the back end.
3) The trainer has their own blog, so I am not sure if I need to create a separate custom post type and taxonomy?

Any ideas?