If you employ a cross-border commuter from France who is liable to withholding tax, you must submit an annual “Attestation de résidence fiscale francaise des travailleurs frontalieres franco-suisses” to the tax office. This certificate prevents double taxation of your employee.
The employee can obtain the residence certificate in her municipality of residence. Your employee is only exempt from the general withholding tax obligation in Switzerland if she has this residence certificate.
An example of what the residence certificate should look like can be downloaded here.