First and foremost, she needs adequate errors and omissions insurance as well as regular business insurance, and a personal umbrella policy for however many millions makes sense. But in the past 30 years I've only done LLCs, which she could do and remain a sole proprietor. Whether it's worth electing to be an S corp is hard to say, but factors to consider are losing the home office deduction, paying for payroll processing and payroll tax returns, and paying for an S corp income tax return.
With a service business, the IRS is going to want to see almost 100% of profit being taken as salary and not distributions.