The principle of charity recommends the latter reading.
If someone has a thin skin, that almost by definition means that their subconscious is running on something opposite to the principle of charity: the principle of erring-on-the-side-of-assuming-that-people-who-say-potentially-hostile-things-are-in-fact-hostile. You can’t just tell someone to apply the principle of charity in that case; that’s applying a counteractive strategy to a deeply felt emotional experience, and as such ineffective.
If someone has a thin skin, that almost by definition means that their subconscious is running on something opposite to the principle of charity: the principle of erring-on-the-side-of-assuming-that-people-who-say-potentially-hostile-things-are-in-fact-hostile. You can’t just tell someone to apply the principle of charity in that case; that’s applying a counteractive strategy to a deeply felt emotional experience, and as such ineffective.