Speaking from experience: once one party realizes there's not going to be a relationship, well, ever. There's really no going back unless both, emphasis on both, parties find value in the friendship.
Odds of the 2nd happening are tantamount to winning the lottery. And if it's happened to you, cherish that friend. You don't eventually find yourself mostly isolated like I have without realizing at least one side of the table didn't want to be there over and over again.
Edit:
Hmm... My diction is kinda wonky at the end there. Not sure how else to phrase it... Hmmm