Dear katy,
maybe they miscalculated her hcg count the first time. If her hcg level was going down that quickly she would be experiencing a threatened miscarriage, but if she is not showing any signs of bleeding, abdomen pain or lower back pain then there shold be no reason for her to worry. The hcg levels should be going up not down. Maybe she could have her hcg levels monitored over the next week, and if her hcg levels are still going down with no bleeding or abdomen pain then her doctor may request an ultrasound.
Good luck, I hope your daughter is doing fine...
Suzy