Is it permissible to supplicate for Allah to separate a betrothed young man from his fiancée so that he may return to the one he used to love, or does such a supplication cause harm to the other girl?

There is no harm in supplicating for marriage to a specific person or that good be destined for him. However, it is not permissible to supplicate for him to divorce his wife. It is advisable to supplicate for a righteous spouse and that Allah facilitates what is good for you wherever it may be, for Allah knows what is good for a person, in accordance with His (the Almighty’s) saying: "And perhaps you dislike a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not."