Assuming she was on an F1 visa for less than 5 years or part years, she would start counting her days of presence in the US in October 2018. She will not be a resident in 2018 as she will be in the US less than 183 nonexempt days.
If you have not already made an election to file a joint tax return with your nonresident wife, you can do so with your 2018 jointly filed return. See below on how to make this election.
If, at the end of your tax year, you are married
and one spouse is a U.S. citizen or a resident
alien and the other spouse is a nonresident
alien, you can choose to treat the nonresident
spouse as a U.S. resident. This includes situations in which one spouse is a nonresident alien
at the beginning of the tax year, but a resident
alien at the end of the year, and the other
spouse is a nonresident alien at the end of the
If you make this choice, you and your
spouse are treated for income tax purposes as
residents for your entire tax year. Neither you
nor your spouse can claim under any tax treaty
not to be a U.S. resident. You are both taxed on
worldwide income. You must file a joint income
tax return for the year you make the choice, but
you and your spouse can file joint or separate
returns in later years.