I was sitting the row behind the exit row with my wife about 3 weeks ago. In the exit row was a mother with her 8 year old daughter. The FA came and said the girl couldn't sit in the exit row. Mother argued something fierce saying she wouldn't move, then tried to move her 13 year old son (who was further up the plane) to switch with her daughter, but he was still too young.
The mother really was putting up a fuss and did not want to give up her seat (the window exit on a 319 with its 20 feet of legroom.) I looked at the FA and said my wife and I were traveling together and would trade with the pax and her daughter. After much cajoling (and some tears from the daughter who didn't want to sit away from mom...) she agreed to switch. She was so mad at the FA that I could see the complaint letter being formed in her mind. The FA for her part was VERY professional and very appreciative of the offer to help (she offered us a free drink, alas, we don't drink.)
99% of the time when I've seen something like the OP saw the FA has stepped in and done something. I've often heard FAs say that they want to be careful about who is in "their exit row" because they know it's those people who may be important in an emergency. However, some people will go to any length to keep their prime seat (or get it - more than once I've caught a squatter in my exit row seat - but that's another thread...)