I agree, legit under the tax law as written.  A counter-argument might be that on the Marketplace application the taxpayer misrepresented that the non-dependent was part of the "tax family".  But nobody has a crystal ball, they may very well have expected the non-dependent to be a dependent when they filled out the application at the end of the prior year so for there to be fraud the government would have to prove intent.  Best I can tell though, everyone sticks their head in the sand and just lets these go.

Rick