Make sure you house them together once they have meant forever, not off and on. The constant departure of their friend WILL make them depressed, anxious, and sad. You will both end up with two very sad and not very playful gliders that will likely begin to overgroom [not a good thing!]. Keep these two gliders together from now on. What you can do is keep the cage at your house one month then hers the next but it is hard for gliders to acclimate so do not move the cages too often [like every night] as this will cause it's own issues.
So glad to see you have read about gliders needing their buddy, it is so true and the best thing you can do for them! Welcome and continuing asking questions because there is a lot of information on the web about them and not all of it is good [like the youtube "vet"].