Can what befalls the fiancé—nightly choking and the appearance of scratches on his body—which ceases with their disagreement and returns with their reconciliation, be caused by his fiancée? And should the fiancée be patient with him in this situation, or should she leave him?

What happened to the man could be due to sorcery or possession by the jinn, as the Prophet, peace and blessings be upon him, was himself afflicted by sorcery. If the suitor is suffering from something of this nature and it is possible to treat it, then your friend should not leave him if he is a man of religion and good character. However, if bad character is a persistent habit with him, then it is safer for her to leave him.