I agree the "I told you so" approach will turn her away, forever locked in her delusional world and right now she seems open. But there's a more subtle way to do it...
The fact is Romney can say all he wants but it won't change a thing. He's a pathological liar which happens to be on overdrive right now. Of course I would not tell her that.
What I would do is explain the various reasons why Ron Paul republicans would never vote for Romney. Such as, "They're very strict constitutionalists and Romney has done x, y & z", or "They vehemently disapprove of Obama's actions/stances on x, y & z and see Romney in alignment with him on those positions", or "Many are libertarians who believe in x, y & z that is incompatible with Romney's positions" and so on.
THEN, I would explain the various reasons why Ron Paul has a better chance at beating Obama and then throw it back at her, "So what would it take to have you help me convince others to demand revoking Romney's nomination so we can fulfill our mutual goal of taking out Obama?"
She'll likely tell you why that will not happen and then I'm betting she'll express the need to move forward with Romney as the nominee. I would then let her know that's unfortunate.
THE FACT IS... if Ron Paul was the republican nominee he'd take Obama out after the first debate - no question. And it really is THEM who are handing Obama another term in the WH. Unfortunately however, republicrats treat their parties like it's their religion. Any direct attack on their religion will cause their closed minds to tighten further. So all we can do is give them something to think about and let them discover the truth for themselves, so they will then own it.
I wish you luck in whatever approach you decide.
Unless Ron Paul! -Lorax