It's Latin, not French.
"Et tu" could be from both languages, but the origin of the saying is what Julius Caesar said to Brutus after being deadly stabbed.
Also, I'm not sure what the usage would be in classical Latin, but using the singular to talk about a company in French seems perfectly appropriate. IIRC it's the Brits who like to use the plural for companies and institutions.
I've never been clear on that.