Depends entirely on how you've implemented your validation. Was it not possible to implement your validation using CB Code Field or CB Query Field? Both provide a means of implementing custom validation. CB Code Field provides Code Validation so you can implement custom PHP based validation and CB Query Field provides Query Validation so you can implement custom SQL based validation. Both also already support ajax validation out of the box.

CB Code Field and its Code Validation provides the user object for you. You can access it with $user from your custom PHP. I've no idea why you're trying to run validation from a Joomla module and don't see how that could possibly work for anything other than client side validation. To properly extend CB validation it should be a CB field as we've triggers in place for extending validation (e.g. see CB Code Field for example). The trigger will provide the edited users user object. Aside from that you'll have to pull the user id from GET and build a user object from that id.