Pax sat next to an emergency exit have to be capable of operating the exit unaided in an emergency. Overwing emergency exits of the window variety have to be removed and then jettisoned overboard so as not to block the exit. They are very heavy and would require a fit and able person to manhandle them.
The check-in agent made an error in this case in allocating the seats. There are several restrictions on seating people in various seats usually for safety reasons.
Hope this helps.