With default settings the ModifiedBy property of the documents (the user who last modified the document) won't be indexed and therefore can't be used in advanced search.

After some hours of investigation without any success I got a solution from MSDN forum posted by Pritam Dahake that seems to work:

1) Open Central administration -> Shared Services -> SharedServices (X being the number of the shared services provider for the site you are creating a mapping)  - > search settings -> metadata property mappings
2) Click "Modifiedby" to edit the properties
3) Make sure  "Include values from all crawled properties mapped" is selected
4) Click "add mapping" and select the "Office" category from the dropdown list.
5) look for a property named "OFFICE:8" , click "OK"
6) Click "add mapping" and select the "ows_Last_x0020_Modified(text)" from the ALL categories dropdown list, Click "OK"
7)  Make sure the box is checked "Allow this property to be used in scopes", click "OK"
8) Click on "Crawled Properties" link on the left side of the Shared ...