You said it yourself that you are not ready to date.
Trust your heart - do not force yourself just because someone is datable out there.
He dates your best friend - I do not know about you but I think it is a little heartless of them to parade it in front of you. Distance yourself from them both until you get emotionally better or until you get to meet your new bf. You do not have to cut contact altogether but.. if you are in too much pain, you have all the reasons to do so guilt-free. If your friend asks you about why you are distant, if she is very important to you, you can honestly explain that although she is your best friend and you are happy for her you are in a lot of pain to see her with the guy you wanted to be with so you need your own time to heal. She will understand. If she is not that important to you anymore... you do not have to explain anything. Courage. From what you shared, it seems like you are better off alone right now because you deserve to meet and date someone more attentive and more mature :)