Levels of Looking at Learning: Cena's fragile knowledge about place value

Keith Devlin writes in his recent blog "How to design video games that support good math learning: Level 4":

"A major problem with video games, or more generally any mechanized educational delivery system, is that the system has no way of knowing what the player, or student, is learning. That a player who moves up a level in a video game has learned something is clear. Video games are all about learning. But all you can reliably conclude from a player’s leveling up is that she or he has leveled up. It could have been happenstance."

The student may not have learned anything significant. Appearances can be deceiving as Keith noted after he watched this video.

Keith continues:

"If you are like me the first time I saw this video, when you heard Cena’s answer in the class you concluded that she understood place value representation. She certainly gave the right answer. Moreover, to those of us who do understand place-value, her verbally articulated reasoning indicated she had conceptual understanding. But she had nothing of the kind, as the subsequent interview made clear." 

Let's review the video above. Here's what happens.

The teacher drew 49 stars and circled 4 groups of 10. When she got to the last 9 stars, she asked the class whether they should be grouped? 

"No," says Cena. "You got uhh one, two, three, four tens. You like put a four right there. And you have 9 stars left over so you put 9 right there." 

To which the teacher replies, "So does everyone understand?"

At that moment Marilyn Burns comes on and says, "Children's understanding is often fragile but what they know in one setting doesn't always transfer to another."

New scenario. Marilyn is now sitting next to Cena with a bunch of tiles on the table. Marilyn continues, "Put the tiles in groups of 10 and count out loud so I hear what you are doing." Cena counts out ten as she places them in a pile. "Can you make another pile?" Marilyn continues. "So how many groups of 10 do you have? "2," Cena responds. how many more do you have? "4". Do you know how many tiles you have all together? Cena responds with "Uh-uh (no)."

How is that possible? This is the same girl that so brilliantly knew to write that there were 49 stars by counting 4 groups of 10 with 9 left over. Keith didn't offer a possible explanation and neither did Marilyn. That, of course, was not their purpose; all they wanted was to make the reader/viewer aware that what you see is not necessarily what you get.  So that begs the question for me. What would you do: (1) as the teacher in the classroom to confirm Cena's understanding and (2) as a tutor ala Marilyn Burns?