Members area to show specific information?

I built a website for an attorney and he asked me if he could have a members area where a client could log-onto the site and view the updated case information pertanent to only his/her case. Basically when the user logs in they could view only their case.

So I am looking for the ability:

The client can register (preferably privately).
The client can then login onto the site.
The attorney (website owner) places their case information on the site (in the appropriate spot).
The client will see this updated case information (that the attorney updates) once they login.

Is there a simple plugin that would accomplish this goal?

You could use MODX user's Extended Fields for this, and use the Login package's Profile snippet to display the user's information. The Login package includes a Register snippet.

http://rtfm.modx.com/extras/revo/login/login.register
http://rtfm.modx.com/extras/revo/login/login.profile