UGH. Debt collector harassing my grandmother!

Hey all.  I'm a new member here, and I am hoping that you can provide some tips on the situation that I'm about to outline here.

OK.  So.  First of all, I'll say flat out that this has nothing to do with MY student loan debt.  It has to do, rather, with the student loan debt of a cousin of mine.  This cousin has been out of school for YEARS, but apparently went into default on his loans or whatever (I honestly don't know the details.)  This cousin has since married and moved across the country.

Anyway.  A collection agency started calling my GRANDMOTHER'S house about a year ago.  My cousin has NEVER lived with my grandmother.  EVER.  The only thing we can figure is that they tried tracking down my cousin and, since he and my grandmother have the same first initial (and my grandmother's phone number is listed under her first initial only), they figured that it was the right number at which to contact my cousin regarding his debt.

They have been told, REPEATEDLY, that the person they are looking for cannot be contacted at the number they have -- and that in fact the only people living at that address is a sick 90-year-old woman and her caretaker.  They refuse to believe this and have even gone so far as to call anyone who tells them this a liar, flat-out.

Well, today I was there when they called, and I answered.  I told them, AGAIN, that the person they were looking for cannot be contacted at the number they called.  The agent then asked, in a very snotty tone, "Well, then, why do we have this number on our files?"  I told him that I don't KNOW why they have that number "in their files" (which, by the way, I don't believe, because as I said, my cousin has been out of school for AT LEAST twenty years, and they didn't start calling my grandmother's home until last year.)

So the agent started going on and on about how defaulting on a student loan is a prosecutable offense and how lying about the debtor living there isn't going to make any difference and blah blah blah, and finally I just kind of snapped.  But I managed to stay calm, and I said, "I would like the full name of your company and the full mailing address."  He gave it to me.

I then told him, "Your company will be receiving a cease and desist letter shortly.  Once you receive that letter, you are no longer to contact this number.  You have been told on numerous occasions that the person you are attempting to contact does NOT live here, and you have still continued to harass a sick, elderly woman.  In addition, I will be filing a formal complaint with Consumer Affairs, in both the province in which you operate, and at the federal level as well."  Then I hung up.

Now, I fully intend to send a cease and desist letter to this company (General Credit Services.)  But what I am wondering is:  the only templates I can find online deal with letters that are being sent from the actual debtor.  Is there anything different I need to do in order to send a C&D letter on behalf of someone who is NOT the debtor?  For instance, I don't have the account number or anything.  I also don't have a specific name to put on the letter.  Would any of that matter?

I would really appreciate any tips anyone could provide.  Thank you so much!