
If I missed it, did the seller say that the skinny female had laid recently? If so, how long had it been since she laid, and how long does it take for monitors to get back up to proper weight after laying? OP, is the animal accepting a normal amount of prey?
I'll say that even if this amount of weight loss is normal for a laying female, it seems like the seller is admitting he knew she was underweight and did not tell the OP about it before the sale, and that's not right.
I'd also like to know if the animal is alert and acting normally, or if she is sluggish/weak.
arta will you be taking this girl to a vet?
Also have you considered offering dubia roaches rather than crickets? Dubias can be contained in a shallow smooth-sided dish, so she could eat without having to expend any energy to hunt.