1120 never filed return (since 2000)

Most states require corporations to file annual reports, or lose their status.  Are you sure it wasn't dissolved in 2001?  

"Foreigners buying Florida homes" is another way of saying "nothing to see here, just ignore those stories about money laundering."  Are they Russian oligarchs, or Latin American drug dealers?  We'll never know, but maybe law enforcement will find out with the help of BOI reporting.  

I looked up how Florida corporations are administratively dissolved.  Apparently if you don't file an annual report, with the annual fee, you get an email telling you adios, до свидания.  You can look up corporate status online.  The problem these "we aren't in business, we just pretend to hide ownership" people have is that when they want to sell the place (and that may be why they are getting around to asking for help) the buyer is going to want clear title.  

SMLLC's weren't exactly prevalent back then.

Have the client retain legal counsel first.  Then, the client can tell you what needs to be done after they have spoken with an attorney.

"no income for the Corp all those years"

Was a return required to be filed? I don't know if the foreign ownership supersedes  the no income or deduction rule for filing. Just throwing it out there.