Hi Josh,
Welcome to the forums. Although I agree with you comletely, you'll have a hard time on the forums finding someone who will tell you to tell you friend that she's wrong. We have to accept that some people will do things with their own names that make no sense, and if they want to use a "precedent" that is 700 years old to justify their silly actions, then we just have to let them. (I could imagine doing this in jest, but not to the point where I expect others to believe this the correct way to pluralize their name.)
Are the members of the Greenleaf family known collectively as the Greenleaves? Is the Tandy family the Tandies? We could, I suppose, have some fun with this.
I can only assure you that your instincts are completely in sync with mine, and hope that you will do better at curbing an urge to roll your eyes than I would in the same situation.