Neither I nor the rest of my friends would even think about throwing a favour back in someone's face, as this woman has done to you. It is fine to accept this woman's point of view and to understand her grievance, but that does not mean that you are obliged to do anything about it. The fact that she confronted you about it in this way, and the subsequent suggestion that you clean your car before daring to help her out again, would leave me thankful that she had "unfriended" me on social media.
I know it's easy for me to say in a "keyboard warrior" type way, but seriously, if someone said that to me I would explain politely and succinctly why I don't find that acceptable. You don't need to worry if she finds this unreasonable or not.