My MIL is accusing another resident of stealing. How can we help her to process this so that it is not an on-going issue in the community?

We don't know what the truth is and never will. That doesn't really matter. The "facts" as reported by my mother-in-law, who is 91, is in a small assisted living facility, and has some dementia and a lot of short-term memory problems, are as follows: She was in the sunroom, and decided she needed to leave to go get something. She says she left about $30 cash on the table where she and other (one? two?) women had been sitting. One of the women is the "new girl," younger but not with as many advantages as Mom. When Mom came back the money was gone. She is sure that the new resident took it. We have tried everything to get her to let it go. The money is inconsequential. Lesson learned, don't flash your cash, even in a small community. But she equally could have dropped it somewhere. (Or, the woman, or someone else, could have made a bad decision and taken it. In any case, if that person either needed the money or could not resist temptation, it's over.) The issue really is, it is affecting the community because we can't get her to remember not to say anything, to the other woman or to anyone else. She has lots of memory issues and normally she would just forget it, but THIS she remembers (or thinks she does) - "So-and-so stole my money." How can we help her to process this so that it is not an on-going issue in the community?