Halloween // Costume ideas for well-dressed 70yo mother in law?

My MIL is invited to a costume party in Feb (Fasching party of some German friends) and we are stuck on costumes. She is not usually one to dress up and all of my own costume ideas and plans don't work for her. She is usually very put together, not necessarily conservative but very well dressed - think Prada, some Comme de Carcon etc.

Any ideas? Googling hasn't gotten me very far, either and without a theme to even start I am very stuck. TIA!