The files will be preserved and will retain the user ID of the owner. Suppose user john has uID 501 and gID 100 = users, after deletion of user john you will see:
Code:
-rw-r--r-- 1 tom users 3342 Jun 13 09:47 file1
-rw-r--r-- 1 501 users 5072 Jun 13 09:47 file2
The only things that will be deleted (unless you decide to keep them) are the user's home directory and the user's mailbox.