The crush

So my one of my close friends told me about a month ago that she had a little crush on this guy. I liked him too but didn’t say anything. Since then he’s been messaging me and admitted he liked me, I like him too and I really would like to be with him. I asked my friend if she still likes him and a he said not really and if I did I should go for it. I know she wouldn’t be mad at me but I just don’t know how to tell her because I don’t know what id say in the message. Help?