Confusing checkboxes and radio buttons

I have a problem building a certain UI for a website. I'm going to demonstrate the problem with a simple website for a restaurant chain:

Context

the website has many users and many restaurants. Each restaurant has certain users assigned to it, where the user could be either a manager, a cook or a waiter.

the waiter can see the addresses of the orders that come in, the cook can see that and the content of the orders (what the food is), and the manager can see both these things and can change the roles of any user.

There can only be one manager for every restaurant, but as many cooks and waiters as desired. Here's how the UI looks like as of now:

^{download bmml source – Wireframes created with Balsamiq Mockups}

Problem

Users can only have one role each, since every role has all the privileges of the previous roles plus a new privilege. So no one can be both a waiter and a cook, or a cook and a manager etc. That isn't very clear using the UI above, since one could check two checkboxes for the same user, or even all three roles.

Question

How do you you make this idea clear through the UI? 'the idea' being the there can only be one manager for a restaurant, and every user can have only one role.

This is my first post here by the way, so I hope I'm in the right place. Thanks for responses in advance!

EDIT: There are higher roles that can change all three mentioned roles freely.

The last case is what I'm trying to avoid (Mary).

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