My girlfriend put her 2010 3.5 Impala into neutral, at 80MPH, by mistake. Would the rev-limiter still work?

Yes.

The rev-limiter always works.

Yep it'll work, in fact I think it will limit the RPM to 3000 or 4000. It'* controlled by the electric throttle body on it. As soon as it sees the limited RPM it simply closes the throttle to obtain that RPM. I know in park if you put it to the floor it'll limit it to 3-4K RPM, haven't tried it in neutral but I imagine it'd be the same.

Thanks ! !