How to delete all files in Pictures folder after I moved all the pics to external HD

Firstly, yes he has to point to the external HD when trying to import new pictures into his computer.

Then you can easily delete Photos Library (see following pic) (usually located in: /Users/<name of the user>/Pictures) that package contains all the photos in the application Photos. At the end if he will proceed with emptying the Trash all the pics in the Mac will be deleted.

More precisely, you can do the same for iPhoto. The library name depends only to the application you are using. So for instance if instead of using Photos, your friend is using iPhoto the library you want to delete will be iPhoto Library!