Is this your inference? When I read that chapter, I immediately googled it and 'polyfluis' to see what it did, but only turned up MoR.

Well, that's the obvious implication of

The round-faced first-year girl stood facing the remaining two bullies with one hand cocked on her hip.

Grinning.

And surrounded by faceted blue haze.

"Polyjuice!" spat the bully-girl.

"Polyfluis Reverso!" roared the remaining boy bully.

Something like the form of a mirrored scarf spat out of his wand -

Passed without resistance through the haze surrounding Susan -

For an instant, she glowed in a strange mirror-color, like a reflection of herself -

And then the glow faded.

The young girl still stood there, hand on her hip.

"Wrong," said Susan.