If you feel the need to tell her be straight forward and tell her but be prepared for questions and its best to have proof to be able to back up your accusations. With that being said you have to ask yourself if it is your place to be the one telling such things to someone to begin with and weigh your options because you have to know you are going to be turning lives upside down and the carrier of bad news. Choose wisely and do what you think you should do.