You are going to have to make it clear to the friend that you were not dating though he may have wanted a relationship with you, you made it very clear to him that you were not interested in him like that. As long as the friend thinks the two of you were dating he will think that makes you off limits and that is why you are going to have to communicate to him your side of the story even if he still chooses not to date you because of the code between friends not to date each others exes. You should also say something to the guy you were talking to and tell him that it wasn't fair that he tried to sabotage your getting to know the friend and make it clear again that there would never be anything between the two of you. He is being delusional and needs to be set straight even before you talk to the friend. If you are bold enough confront him in the presence of the friend and ask him "Did I not tell you that I wasn't interested in a relationship with you and only wanted to be friends?" He may not have a choice but to finally tell the truth and even if he lies about it the friend will know that you are telling the truth because you wouldn't confront him in front of him if it weren't the truth.
You're quite welcome and let me know if you need my help again. Good Luck