Dispute.

[Talonblaze]

And I'm saying it could be seen either way. I get where both you and your friend are coming from. And if you look at the Sudden Metamagic feats (considered Metamagic feats), you'll see they are functionally identical to the Metashadow feats. While the wording of the Metashadow sidebar leaves a bit to be desired and obviously is unclear enough to allow this dispute, looking at various pieces of evidence and alternatives is still warranted.

The key thing your forgetting is that even Sudden metamagic feats tell how they affect spell slots. Even if it is none. Shadow Metamagic only has the one entry referring to their spell slot adjustment and that is the line saying they function exactly like the metamagic counterparts. No where does it say their spell slot is not adjusted with these.
Text written > Text not written.