First off you want to be sensitive to how you approach her. Don't use terms such as "bad teeth"; "rotting"; etc. State you are concerned about her oral health and ask how you can help. She may brush you off, be angry, and put up a wall. But keep gently asking about it from time to time. Show you want to help, offer to talk about it, and ask what she wants done. Involve her in the decision making process.