To quote @ChrisInEdmonton:
If she is a U.S. citizen, she needs to worry about U.S. taxes regardless of whether or not she is living in the U.S. She must file a tax return even if she spends 0 days in the U.S.
It's also worth pointing out that you need to declare all the moneys you made throughout the year wherever they came from.
But there is a another part involving worrying about taxes and that's whether or not you are going to have to pay additional taxes on the money you made abroad.
If she absolutely wants to make sure that she won't pay additional taxes she needs to qualify for the Foreign Earned Income Exclusion. It lets you exclude up to ~$91,500 from your taxes from money that you've made in another country. As far as I can tell it doesn't matter if you didn't pay any taxes on that amount.
There is a nifty question and answer form to see if you qualify, but the basic take away is:
You qualify IF:
You were bona fide resident of a foreign country or countries for an uninterrupted period that includes the entire tax year.
You were physically present in a foreign country or countries for at least 330 full days during any period of 12 consecutive months
To me this all means that In the worst case situation, in any tax year, the maximum number of days you can stay in the US is 35 days
Even if you don't qualify for the foreign earned income exclusion (for example if you just started a job half way through the year), you still may not have to pay any additional taxes. Instead you can take a credit or deduction for the taxes you already paid in the foreign country.
To me this means that if the foreign tax rate is higher than the federal taxes rate, you don't have to worry but if it's lower, then you will need to cover/pay the difference.
Note: I'm not a tax professional. Take everything I've said above as completed fiction until you check with someone who knows what they are talking about.