Just because the little boy did not test postive for giardia does not necessarily mean that he does not have it. Giardia often does not show up in the first or even first few fecals.
Do the gliders play in the same area? are the cages nearby? or you wear the same thing (pouch/shirt/etc.) when playing or bonding with them?
I have dealt with giardia with my gliders and it can be a pain to get rid of. I had two cages of gliders that did not ever come in contact with each other - but did play in the same area. In one cage - only 2 of the four showed signs of giardia (one being a little joey who passed away). In the other cage - the other two also showed signs but never tested positive. All of my gliders were treated.
If you just got your little girl - I would suspect that she had the giardia already in her system and then the stress of a new home allowed the parasite to flourish. Giardia though can come from many sources. When mine got it - they had been living with me for a year, got water from bottled water, and all of their fruits were washed and frozen...but somehow something must have slipped passed me.
Be sure to keep everything sanitized. I bleached all toys, cages, bowls/water bottles, etc. in a hot water and bleach solution. I personally erred on the side of caution and cleaned everything every day and changed out pouches every day.
What meds did the vet prescribe?
Often flagy is prescibed. Be sure to watch for any neurological problems (dizziness, bobbing of the head, etc.) or jaundicing of the skin that could signal trouble. Also - be sure to push a lot of fluids with meds and it can be quite hard on their systems. I also offer yogurt daily. SOmetimes the gliders will not eat as much - so you will need to carefully monitor her food intake and weight to be sure she is eating.
My vet used a flagy and panacur combination to treat...and more than one treatment may be needed to ensure that the giardia is completely gotten rid of. I had to do 3 rounds of treatments with my gliders.
they should be brought in for a fecal to re-test after meds have stopped - and I personally took fecal samples in every couple of weeks for the next few months. Giardia is not shed in every stool - so it can be difficult to get a positive fecal even if it is present in their system.
I can check the board for old posts of when I went through giardia with my gliders and post links if needed.
If you have any questions - ask and I'll try to answer what I can.