An Employer Identification Number (EIN), also known as a taxpayer identification number, is a nine-digit code assigned by the IRS to identify a business. You can think of it as the social security number for your business. An EIN is required for a partnership, corporation, or LLC to open a business bank account, obtain a loan, or hire employees.
Without an EIN, having to combine business and personal funds increases liability and puts personal assets at greater risk. I highly recommend getting her federal EIN number and opening a separate business bank account so you can distinguish between business and personal transactions.