I will PM you.
Oops. It's long. Goes well with caffeine...
If anyone is wondering, the gist of it is stuff she may already know, like taking really good care of yourself, taking deep breaths in the toxic moments, remembering the issue is part of a larger more manageable problem ("working with idiots"), finding out if others have had issues with the creep (union rep, HR, other employees in the past, or even the creep's residential neighbors or possible group home manager, guardian, or conservator). Complaining to the union or health and safety at the state branch of Labor and Industries. Baiting the thief in order to video the crime. Baiting the bag with incriminating contaminants. Providing the thief with a charming bag for herself and then RAIDING it later, ha ha ha. Stealing the thief's lunch (I am a gentile not subject to the 10 Commandments). Confusing the thief by offering her meal bars and a drink when her lunch goes missing. Stealing pens or supplies with plausibly urgent need right under the nose of the Judas coworker. Throwing a fit and raging, towering over the thief's desk and pounding on it. Calling the cops to the workplace (if truly ready to get a new job). Rating the boss on Bosswatch. Diligently waiting for opportunities to document, expose, and get back at the thief. Protecting her bag in another bag as Anniem mentioned. Or letting it all blow over and going underground in these efforts. And asking management for a portable personal locker.
