What shall i do about this boy & my friend?

okay i have gone through the same thing and just let them be ..if they go out well just tell her 'why would you do that to me you know i like him' and see what happens if she says something like "it isn't a big deal" say " well it is to me! cause you lied to me and said you wouldn't do anything" that's what you do and then let life pass by and if turns she keeps on going out well whatever what can you do.