Possessive apostrophe, for heaven' sake!

Through reading "When Words Collide," I didn't expect to learn anything new, having been working with the AP Styleguide and writing extensively for about five years.  I was surprised to learn that when using an apostrophe as part of a possessive noun, "If the next word begins with s, add an apostrophe only."

I use phrases like "for goodness' sake" and "for heaven' sake" all the time when speaking, but never when writing, so it hasn't ever crossed my mind to think that "heaven's sake" looked funny. 

I also have always struggled with apostrophes after an s.  I remember from grade school the fact that there was a rule for them, but I have always struggled remembering what it was.  Apparently it’s not as difficult as I thought; if there’s an s at the end of the word, apostrophe only.  Go figure. 

Now, I’m always on Facebook, and I like to think that most of my friends are well-versed in the ins and outs of punctuation.  So naturally, when I find a mistake, I cringe.  My uncle posted a picture of my cousin and his friends the other day, after they had finished running the “Tough Mudder” race where they live.  The post looked like this:

 Now, I’m conflicted over his use of a comma in that sense.  I could argue that he is referring to my cousin and his friends each as a “tough mudder,” but, honestly, that feels like a stretch.  I think he meant to say, “They just finished running Tough Mudder!”  At least I hope that’s what he meant.