Why 1040 and not 1040NR, since she is a non-resident? Because she is a US citizen not residing in the US. 1040-NR is for nonresident aliens.

Would she have to file FBAR's for all the 20 or so years?  Technically yes. There are some provisions for late filing of these where they accept 6 years. Penalties are huge so research and legal advice are in order,

How could she file MFS?  Her spouse is a French resident and doesn't have a SSN or ITIN. Some software allows "NRA" in the spouse SSN field for this situation.