If there isn't any animosity between the two species, but an indifference or an alliance, don't you think they would look out for each other when it came to the humans entering their reality? What I mean is, if Moiraine knew from them that the only way she could be saved would be by Matt, then they (the finn) should have known that a rescue attempt would be made, and that it would compromise their safety somehow. Er...wait, as I am writing this, I am suddenly unsure as to how Moiraine figured would she could be rescued: was it through the portal to the finn's world in Rhuidean, or was it through the other ter'angreal similar to the one in the basement of the White Tower?

Even if Moiraine didn't find out from the Finn, the Foxes can see through Matt's vision anyway, and they know that he will be cheating when he arrives...