They will transfer you to "retention" just say no to everything. No matter what credit cards you remove they can charge/bill you on any preexisting charged card which is part of their agreement in the contract. MANY companies with subscriptions do this. . DO NOT tell them you are not the account holder (or a authorized user on the account) or they more than likely will not talk to you (they are not supposed to). Nothing wrong with following up in writing but honestly it won't do much.
I say this from experience both as a long time former customer and a CALL CENTER TECH SUPPORT TRAINER for 5 years in their MDU division, which I started out in retention training. This goes back probably 15 years but they have not changed. "JUST SAY NO" over and over and make sure ANY EQUIPMENT is returned. If they tell you to keep it (they do with some old equipment). DO NOT JUNK IT FOR 2 YEARS! Your account is not cancelled until they get their equipment and you WILL BE BILLED for the equipment if not returned.if they ask for it and sometimes even if they don't ask for it. If you do return it make sure you take photos of all the numbers on the backs of each box and make sure you get a receipt for the tracking. Their contracts have lots of fine print.
Make sure you call them DURING THE DAY so the chances are less you will be transferred to offshore. If your friend is still under contract ONLY DEATH (certificate required) or moving to a location where there is no service or being deployed overseas (with proof!) will get you out without paying the $20/month ETF. I would suggest your friend adds you to his account if they still do that.
Good luck. Remember: JUST SAY NO !!!!
. Hopefully you have no hassle. SInce they were bought by AT&T it's only gotten worse I have heard.