Lost her entire inventory? She didn't get sent to prison did she?
If you get caught doing something and taken to prison your loot will be put in a nearby chest as you are in jail.
Migo's Ring can be given to a certain npc for experience, I don't know if another person can turn the quest in while they are not in possession of it, but perhaps you did without realising.
Other than that I have no idea, I know allies can pickpocket you with thievery, and I have never had a disappearing loot issue when I have played.