Is it permissible for a woman to refuse her husband's intimacy if that might lead him to commit a grave sin, such as missing the Fajr prayer due to not performing ghusl before Fajr, given her awareness of the angels cursing a woman who refuses her husband?

Neglecting prayers and delaying them beyond their prescribed times is a major sin. The wife should help her husband obey Allah through kind words and good counsel. If it is possible to agree on a time for performing prayers punctually, that is best; otherwise, she should advise him, admonish him, and be gentle. If he does not respond, he bears the sin, and she is not permitted to refuse him intimacy. There is a difference between abandoning congregational prayer and abandoning prayer altogether (i.e., not performing it within its prescribed time), the latter being a greater sin.