If she uses the phone 80% for business (for instance) you can deduct 80% of the cost of the phone and 80% of the monthly bill as business expenses.
When you enter a business asset, TurboTax will ask the Cost and the Business Use %.
However, you are correct that you can deduct the new phone as a one-time Business Expense, rather than setting it up as a Business Asset.
In this case, you would enter the business portion of the phone cost in the Business Expenses section (see screenshot).
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