Is the حد (prescribed punishment) for slander to be carried out against someone who sent text messages via phone containing slander, insults, and immoral language, if no one saw them except the slanderer and the slandered, without the presence of witnesses?

It is not permissible to violate a Muslim's honor through slander or other means, nor to harm them through insults or similar actions. A Muslim's sanctity is greater in the sight of Allah than that of the Kaaba, and "all of a Muslim's blood, wealth, and honor are sacred to another Muslim." What the mentioned person did, whether slander or insult, is a heinous sin, and they must repent and seek forgiveness. The slander that incurs the prescribed punishment (حد) is denying lineage or accusing someone of adultery or sodomy, and it is only proven by the testimony of two just witnesses or by confession; it is not proven by writing. Furthermore, religious punishments (الحدود الشرعية) are only to be carried out by the ruler or their deputy.