The first step is to put the User's Resource in a Resource and the User in a User Group, then connect the two with a Context Access ACL entry for the 'mgr' Context.
The User's User Group will also need a Context Access ACL entry for the 'web' Context (assuming that the resource is in that context) in order to be able to see the resource in the tree.
Then, you'll need to add all other resources to a Resource Group (I call it AllDocs) and connect that Resource Group to the Administrator User Group with a Context Access ACL entry with a context of 'mgr'. The will keep the user from seeing the other Resources. I think you can use the Batcher Extra to do that more conveniently.
You can use the DefaultResourceGroup plugin to add future Resources to the AllDocs group automatically.
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MODX info for everyone:
http://bobsguides.com/modx.html