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If your spouse is not a US person (citizen or green card holder) then she is not liable for tax on her earnings.
Foreign citizens working in the U.S. for foreign governments and international organizations are generally exempt from U.S. income tax and self-employment tax on compensation. This means that even if you file a joint return her earnings are not included. Foreign citizens receiving U.S. source income other than compensation (such as interest, dividends, rents, royalties, etc.) are generally subject to U.S. income tax.If your spouse stays in the US and still works for the foreign government she will not need to report her income form that source to the US for tax reasons.
As a US citizen the rules are a little different. She can exclude her income if she is in the other country for 330 days in a tax year. So the 30 days your referring to is the time that she would be limited to in the US. People try to simplify it by saying that not to be in the US for more than 30 days keeps you compliant with the rules for Foreign Income Exclusion.
Although your wife still needs to report her income on her US tax return, as long as she is in the other country for 330 days she can exclude the income using Form 2555. She has to file but she can show the income then subtract it.