I don't think the answer is clear cut. It's not income for you, but it might be income for your wife.
Anything your wife's employer provides her in exchange for her services is taxable income, unless it falls into one of the limited categories of tax-free fringe benefits. For example, my wife received a gift card from her employer for flowers when her mother passed away, and the value of the gift card was duly added to her W-2 income and subject to income and social security tax.
The employer can pay for a business trip if it is related to business, and they can deduct the cost as a business expense. However, if the employer said "stay an extra week and take a vacation on me" then any of those costs would not be deductible business expenses and would have to be included in the employee's W-2 wages. So I think it is likely that under the rules, the value of spousal travel would have to be included in her W-2. But that is something the employer should take care of. The value of the ticket is not something you independently report as income because you don't have a relationship with the employer.
