Where the Green Lady of Perelandra was most vulnerable was at the point of wanting to experience what she had yet to know. Weston, the demonic tempter of the Lady, was putting into her mind possibilities of what she could experience that were worded to be much greater in value than what the she had already known. Of course it was a lie. She had everything she needed and faith to trust in God that it was true that she had everything she needed.
Weston's brilliance and thus the devil's brilliance, was to try to get the Lady to believe she could please God by displeasing Him. The temptation was toward autonomy. When she would act like her own god then she would be like God. Of course it was a lie. If she acted autonomously then she would be like the devil and not God. What she needed to be was content that she was who God created her to be and was at her best when she was like herself.
The "forbidden fruit" was living life on the fixed island. The only way to get the Lady to want to focus on what she could not do as opposed to what she could do was to appeal to her autonomy and her curiosity to know what it would be like to live on fixed ground. A large part of the sin of discontentment is coveting. A person becomes discontent when he/she allows himself/herself to covet. Coveting can occur when we want our neighbors house or our neighbors life. Once the sin of coveting has taken hold everything needs to be different than it is. But notice how that can never be satisfied. A discontent heart is never settled or at peace because it is continuously searching for something different which is a never ending search.
The repeated warnings in the Bible against discontentment are found in the wisdom literature where we are taught to be content with the things money cannot buy. When the Green Lady found contentment in the abundance God had given her and the King God had chosen for her the voice of Weston was stopped.