1. Clarify boundaries: Communicate openly with your best friend, point out any inappropriate behavior, express your views on healthy interpersonal relationships and correct values, and make her aware of her own changes.
2. Maintain distance: If she still hasn't changed after trying to communicate, consider reducing contact with her appropriately to protect your mental health and moral values from negative impacts.
3. Seek support: Share your feelings with other friends or family, who may provide different perspectives or advice, as well as emotional support and understanding. If necessary, you can also consult a psychologist for more professional assistance.