I have a Verizon MiFi 5510L, and between powering it on and shutting it down, the device used 10gb. My partner has it during the day at work, and I know the only thing that it's being used for is email access and a blog. I've checked the computer that connects to the MiFi, and there isn't anything that would connect out of the ordinary.
It is very easy to consume 10 GB of data in one day. All you have to do is watch a few HD movies, download patches or updates to a major application like an online game and poof, there goes your data for the month.
What do the data logs from VZW show you about the usage? Was it consumed during the time your partner had the device or during the off hours?
The problem is that data usage does not have to correspond to a conscious action by the owner of the Jetpack. Anything that is connected to the Jetpack is able to download or upload content with or without your knowledge. Even if you are not aware of the leakage you are still liable to cover it unless you can prove a defect in the Jetpack *hint - this is very difficult to do.