16 hours plus a 2 hour extension is 100 % legal in certain conditions , just had his happen at UAL for the first time in my career , 117 does allow for it. It's basically your max duty time from table B , plus always having the ability to do a 2 hour extension, hence 16 + 2 = 18 , Under certain conditions as a reserve, it's legit , I called all kinds of people cause I didn't believe it my self , then accepted the extension, called alpa and other people in the know the day after to tripppe check this .

It's legit under certain conditions , however I couldn't speak for what happened in your FO's case.