yes. he is being ammature. he did not say if he moves and she gives him flack then bringing up its a unlawful apartment (wich is what anyone would do).
he said "BTW, I have a great card to play but don't know if I should play it now or wait. I called the city urbanism dep. and they told me that in the area we're in, it's not possible to have a house with a second appartment in the basement or elsewhere. So, I could get the basement appartment closed with just a call...but it's more like a last resort move..."
wich is more like a little kid getting pissy on the playground because the other kids wont play how he wants so he is takeing his ball and going home.
I consider how he said it int he OP very immature.
is the landlord inconsiderate? yes. but he has no real options besides moving. his "card" is nothing more then a retaliation and immature act. Just find another place to move and tell her why. she gives him any shit (or keeps the security deposit (and he does not damage anything)) then yeah turn her in.