Did he try to change her BCD while in the water? That seems like it would send up a huge red flag in something I would not do. "Hey you are not sinking due to a defective BC, so go ahead and swap me in water...".. Nah lets go to shore and figure it out.
To be fair... if it was her last dive of certification, a "remove and replace BCD at the surface" should be a simple skill to handle. It wouldn't be unrealistic for the instructor to assume they could both remove their BCDs, swap, and put them on at the surface.
If this was a case where she was wearing a weightbelt, removed her BCD and sank, then she was grossly overweighted. A diver + wetsuit + weightbelt should be pretty close to neutral if properly weighted. It'll be interesting to see if the details come to light.