The hardest part about moving on is letting go.
The only thing you can do is point out some key realities.
Unless your friend is ready to stop "romanticizing the past" and (accept the fact that it's over) without holding out hope for a reconciliation they won't be able to move on.
In order for him/her to be "the one" they would have to see (you) as being "the one". At the very least a soul-mate is someone who actually wants to be with you!
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde
If someone doesn't want to be with you or they mistreat you it's clear they don't think you're all that "special".
“Some people think that it’s holding on that makes one strong; sometimes it’s letting go.” — unknown
Thankfully we live on a planet with over 7 Billion people on it! That man/woman you feel you can't live without just know this; there are billions of people doing exactly that every single day!
In fact (you) use to be one of them before you met him/her!
“I don’t miss him, I miss who I thought he was.” – unknown
Take down your rear view mirror. Your future lies ahead of you.
Every ending is a new beginning!