That's pretty strange, indeed. I have certainly jumped around between my account and my wife's, always logging out of one and then logging in to the other, and not doing obsessive things like restarting the browser ro clearing cache, and never had such a problem. Including when purchasing tix or buying with miles.

You're right, you need to deal with it while it's hot. I hope it's not a big hassle but you may need to get your friend involved as they can be pretty cautious over the phone about whether you are or are not the account holder.