If your plans are to set up life together in France then there are several things she will have to do before December to be sure she is living here legally before the cut off date. At present if she is not registered for taxes and social charges here, and didn’t submit a tax return this year, then she is not here legally.
Look at this thread, and particularly the URSAFF guide. Her agency will have to do some admin, which sounds more complex that it probably is. But will be necessary to get legal status.