Cancellation of VISA

employer is asking her to bare the exp of exit ticket. That is Ok

No, That is NOT OK unless you, yourself consider it as OK. Legally and as per the law, your employer is bound to provide the exit ticket.

he says you have to fly only through Oman air stating that it is a rule.

There is no such rule. This is hilariously insane. You need to ask him to provide you with the statement of this new rule that he refers to (Which practically does not exist).

Make sure your wife gets a clearance of all the due expenses that may include gratuity, balance salary and other related stuff prior to your departure.