What to do about someone who won't brush teeth?

I really need some help. I have a client who won't brush her teeth. I have tried everything I can think of. She just looks at the tooth brush, I know she does not understand anymore. When I try to do it for her or get her started she closes her mouth tight and refuses. Others at the home she lives in have tried also. The best I can do for her is give her gum to chew. I am really concerned about her oral health but what can I do? She has had dementia for 20 years and I know before I started caring for her she never brushed that often. Anyone have to deal with this? If so any suggestions? Thanks all.