So sorry that one of your gliders passed away. I know how difficult that can be. In addition to getting a scale and keeping track of her weight, I would suggest taking her to an exotic vet for a wellness exam to make sure there's not a medical reason for her weight loss. It's normal for a gliders weight to fluctuate a bit, but a significant loss or gain really should be addressed. I've been through this with both of my gliders and usually it's a simple fix like modifying their diet. It could be she's still dealing with the loss of your other glider and it's affecting her appetite. Has she been eating well? Are your 2 newest gliders in the same cage with her?