The work for the company that gives her a W-2 will be a regular job, separate from her freelance editing business. They will be her employer, not a client, and she "technically" and actually IS their employee. This should not be a problem. A lot of people have both a regular job and self-employment income.
TurboTax Self-Employed (now called Premium) will handle both the W-2 income and the self-employment income. You enter the W-2 the same as any other W-2. It does not go on Schedule C with her business income.
One disadvantage is that a W-2 employee cannot deduct expenses. If she will have expenses related to the job, she should discuss with the prospective employer how to cover the expenses. They could just pay her more to compensate for the expenses, or they could pay the expenses through an "accountable plan" where she submits expense reports and gets reimbursed. The advantage of an accountable plan is that the reimbursement is not taxable. Additional pay is, of course, taxable income.
