Etiquette: Effective communication problems. What would you personally do in this situation?

I work in an open plan office with 10 individuals on my team. I get on pretty well with the majority of my colleagues. My manager is frequently away and asks the most senior member of the team to deputize for her in her absence. This colleague is pleasant enough but very frustrating to work with. My Manager is very specific that if there is any concerns or any changes that need to be authorized to go to her. Michelle is quite sociable to the team until there are work related matter whereupon she becomes very introverted, assertive of her own position, passive and blameful. For example on Thursday, I noticed an error with some formula on an excel spreadsheet and alerted Michelle. I advised her the error was down to a formula miscalculation and asked her to correct it as I wasn't authorized to do so myself.. She advised me she would look at this error at lunch and rectify it. On my return, it was still not rectified. At close of buisness, I asked her if she'd had time to sort out the error whereupon she immediately retorted no, that she had forgotten and would look it tomorrow as she was "up to her eyes". The next morning, she completely forgot anything I had said to her and advised another colleague that she had noticed an issue with the formula and said to the advisor "I have found out what's wrong with formula" as if i'd never told her. In the meantime, an additional error had occurred as a result of her taking such a great deal of time looking into a very simply problem.