Is the questioner sinful for narrating her friend's story to another with the aim of offering advice, and is she sinful for distancing herself from her and not repeating the advice?

You must be cautious about romantic relationships because they cause tribulation and corruption. If your informing your friend about the girl was for a beneficial purpose, then there is no sin upon you. There is no sin upon you for avoiding her and abandoning advice that is useless. It is better to be wary of befriending such a girl, lest she influence you, due to the saying of the Prophet, peace and blessings be upon him: "Do not associate except with a believer, and let no one eat your food except a righteous person."