No, you didn't fail. But, you can challenge her every time she makes an absolute statement.
It's easy reflecting upon what you could have said and done after. But, just carrying any conversation with her about such matters, such is more a success in and of itself than not having any sort of interchange. People can often be influenced through merely knowing others who they know think different from them.
You could have asked her, and what is "the heart" that is always tells us the truth? Has she ever not been led astray by her heart? Plotted to do wrong things in her heart. You'll have another opportunity to continue your conversation should you desire to. Or, to not put her on the spot apply it to yourself like "
I don't know about you, but my heart often lies to me in its desires and passions. I've done many wrong things I know to be false and wrong by following my heart alone."
But, just carrying out a personal conversation is often enough to challenge, and vice-versa be challenged. Seeing your own weaknesses should embolden you to better equip yourself. New Age types are often very emotional, mixed up and confused, I think -- so the better tools to use are your heart, empathy and draw from experiences, and to a much lesser extent rationality and logical arguments.
There is
a good course as Stands to Reason which teaches some tactics to use in discussion with others. Especially, getting out of the "hot seat", putting them in the "hot seat" and giving you time to think and develop your own responses accordingly. Whenever I feel in the hot seat, feeling an attack, I get the person to explain themselves more, clarify what they mean by this or that in their question (e.g., you could ask them to clarify exactly what is "the heart"? and go from there). Sometimes that's where it stops, because they become unstuck in merely explaining themselves and what they mean by their question. Other times it runs into a much deeper conversation, and if done right, they won't feel threatened. Listening, means you get them to think through their beliefs, while putting a few stones in their shoes with questions that will have them thinking long after the conversation has ended.