A couple of little theories!

[justcarol67]

No: HPFGUIDX 162316

kmalone1127:
> > I've wondered about the Foe Glass thing myself. Why would Snape
show  up in them if he was truly on Voldemort's side?
> 
> 
> montims:
> Well, as it's now Barty Crouch's foeglass, Snape IS his enemy - he
betrayed LV in Crouch's viewpoint.
>
Carol responds:
IOW, Barty Jr. was *Snape's* enemy because Snape betrayed Voldemort.
But that's not the reason that Snape showed up in the Foe Glass. He
showed up because he was *Barty's* enemy even before he knew who Barty
was--not because Snape was "a Death Eater who walked free," which is
Barty's reason for hating him--I doubt he knows that Snape was spying
for Dumbledore, only that Snape wasn't sent to Azkaban) but because
Snape was the enemy of the Voldemort supporter and probable DE who had
disguised himself as Mad-Eye Moody and placed Harry's name in the
Goblet of Fire in order to send him to Voldemort. 

The Foe Glass sensed Snape's enmity toward its new owner, not its
owner's animosity toward Snape. Snape represented a danger to Barty
Jr. or he would not have shown up there. He, like McGonagall (for whom
Barty Jr. had no particular animosity), was loyal to Dumbledore and
would do what Dumbledore wished to expose Barty Jr. and to thwart his
goals of killing Harry and restoring Voldemort to power. We see
Snape's surprise at the identity of Barty Jr. but no hesitation on his
part to follow Dumbledore's orders regarding both Fake!Moody/Barty Jr.
and Voldemort, including one order (a request really, and part of a
plan between DD and Snape) that could lead to Snape's torture or
death, the return to Voldemort. And we see Snape voluntarily reveal
his Dark Mark to Fudge as proof that Voldemort is back. Those actions
validate the reflection in the Foe Glass: he is Barty Jr.'s enemy and
therefore Voldemort's. (Note that Harry notices Snape's reflection in
particular as well as the three together but fails to recognize its
significance).

Carol, pretty sure that the Foe Glass, like the Dark Mark gesture in
GoF and Snape's hand gripping his chair when he learns that Ginny has
been taken into the Chamber of Secrets in CoS, is strong evidence that
Snape's loyalities lie with Dumbledore