Alter a field by select list in entity form
A good place to start is the aJax example module, and specifically the dependant example.

In that example, the seconds list is updated when the first list is changed, and it's value it updated according to its helper function _ajax_example_get_second_dropdown_options($selected) which is passed the value of the first list.

You could do something similar except use a simple text field for the second field but place the helper function on default value or value :

'#default_value' =&gt; _ajax_myentityfieldquery($selected),


and in the helper _ajax_myentityfieldquery return the result of your EntityFieldQuery function.