Here's a silent ringtone - if you are on 1.1.2 you will be able to double click on the file (after unzipping) and it should go into iTunes and then sync to your phone to assign to the mother-in-law's address book entry.
Been trying for the last few hours to get this done, with no success. Check out your entry on the forum, and it's done in seconds. Saves lots of potential in awkward incoming phone calls ringing ?!?