What is the ruling on one who mentions the Prophet—may Allah bless him and grant him peace—upon seeing something forbidden, such as saying, “Prayer upon the Prophet” upon seeing an immodestly dressed girl?

It is not permissible to invoke blessings upon the Prophet (peace be upon him) when seeing an unveiled woman, as if approving of her wrongdoing. Invoking blessings upon the Prophet (peace be upon him) is one of the most excellent forms of remembrance, and it is not permissible to remember Allah Almighty in the presence of disobedience. Jurists have explicitly stated that it is disliked to invoke blessings upon the Prophet (peace be upon him) out of astonishment at something, so how much more so when that which causes astonishment is something reprehensible in the Shari'ah. Ibn al-Hajj says: Indeed, invoking blessings upon the Prophet (peace be upon him) should only be for the purpose of worship and drawing closer to Allah, and not out of astonishment. Therefore, it is not permissible to invoke blessings upon the Prophet (peace be upon him) when delighting in gazing at an unveiled non-mahram woman; rather, she must be enjoined to abandon the wrongdoing.