How can one get rid of thoughts about forbidden and deviant sexual relationships, especially since these thoughts are imagined with a close female friend? Is she the cause of this, and how can one get rid of these thoughts without distancing oneself from her?

One must understand the cause of the ailment and present oneself to a specialized doctor, for it might be an organic disease. One must also struggle against the satanic whispers (waswas (definition)) by repelling them and occupying oneself with what is beneficial, staying away from anything that provokes them, such as means of communication and bad company. Adhere to righteous deeds, for "indeed, good deeds do away with evil deeds." Repent from the sins committed, do not despair of Allah's mercy and healing, and focus on studying and elevating one's thoughts to noble matters.