What is the legal ruling in Islamic law regarding a married person who practices masturbation and engages in sexual relations online with girls, despite observing the obligatory prayers, reciting the Qur'an, and repeatedly feeling remorse and desiring to repent?

What your friend did is one of the great abominations, and it is feared that his deeds may be invalidated if he does not rectify that with sincere repentance. He must immediately cut off his relationships with those women and frequently seek forgiveness (istighfar) and supplicate (dua). He should also increase in good deeds that expiate sins, such as prayer, remembrance (dhikr), and charity. Whoever is afflicted with any of these filth must conceal himself, and it is not permissible for him to mention it to people; otherwise, this would place him among those who openly commit sins.