Is the husband's indifference to passing gas (farting) in bed, despite it causing distress and aversion to his wife, considered a violation of the etiquette of marital relations, and what is the ruling of Islamic law on that?

Intentionally passing gas in front of people is considered repulsive and unfitting for a Muslim. Anyone with an excuse for doing so should do it away from people, and a sick person should seek treatment. If the husband does this, he is doing something wrong, and the wife should advise him gently, kindly, and patiently.