Are you trying to cheer him up? Is that the help you want to give, or are you trying to still get her to go out with him? There is really nothing you can do, but be there for him and be his friend. You can tell him "there are other fish in the sea". That's an idiom we use. With so many fish, why cry about the one that got away. I guess that's not very nice, but that is an old "saying" we used long ago to help cheer ourselves up when we "got the cold shoulder" from someone we wanted to go out with.

You can't force someone to like you, simple as that