You can't. Are you implying that when she logs in, she is in a prior period from other users in the same database? The reason I mention it is that accounting period is a database-specific setting, not a user-specific one, so it's not possible for users to be in the same database at the same time and be in different accounting periods to each other. With the changes to the Account Reconciliation window in (if memory serves, 2013) the accounting period setting now only really determines the default "period" setting for reports and the beginning point for the security setting "Transactions in Prior Periods." If the user isn't set to "View Only" for the Transactions in Prior Periods security setting, then there's nothing preventing them from entering or viewing a transaction in a prior or future period anyway, making the changing of the accounting period merely a convenience thing.

In any case, see the following articles for more information:

How can I be sure my two computers are using the same company data?

Can multiple users work in different account periods?

Do I need to change the accounting period?