I don't think it would hurt to tell her. I'm sure she would understand, especially if you told her you were doing it for the benefit of both you and your mom.

If it were me, I'd probably tell my mom too, in honesty. Based on what you posted, I know my relationship with mom isn't the same as yours, and I am limited based on my own experiences and the information your provided. But I would think she would find out anyway, and it's usually better to tell her upfront than be caught. Furthermore, if she's already insecure, I don't think it'd help her to do something (even something as innocent as prom) behind her back. She won't see it like you will, based on your post.

That is just as I see it, at any rate. I do hope I was of some help.