Boyfriend's mother is a manipulator, how should I confront her nicely?

I have been with my boyfriend for four years. He is 26. His mom goes to extremes to manipulate and control his life. He constantly argues with her about her getting into his business but she finds ways to continue. For example, she use to always open his bank statements in the mail to see how he was spending his money, but now since he doesn't live with her, he just discovered that she has created an online account with his bank, under his name, to see how he spends his money. She is a silly old woman who thinks it's cute to do things like this. She thinks he is her husband and while he always tells her about herself, she doesn't stop. How to I address someone this adamant in controlling her sons life because if he asks me to marry him I may not address her too nicely so how do I deal with a professional manipulator? Do I play passive aggressive games with her or straight up tell her how I feel. She is a sweet person and does a lot for a lot of people but my boyfriend is her only child and I think she is afraid to let go but come one he is 26!