If she is not their employee, meaning they will not be withholding federal income taxes, Social Security taxes and Medicare taxes from her income, then they cannot issue a W-2. If they refuse to issue a Form 1099 and pay her in cash, check or other means she can still report the income from this client on her Schedule C without a Form 1099.
Rules for being an employee or independent contractor (IRS) - https://www.irs.gov/businesses/small-businesses-self-employed/independent-contractor-self-employed-o...
