I have to wonder if Zod didn't happen to intercept the same distress call as Rhea from Mon-El--peaking his interest? Or perhaps Rhea recruits Zod (knowing his hatred for the House of El)?

And I'm thinking Rhea misreads Lena. She presents Lena with "an offer," probably assuming Lena is one of "those" Luthors....not aware of the close relationship between Lena and Kara/Supergirl and Lena trying to be a "different kind" of Luthor. I don't foresee Lena easily betraying Supergirl...especially not for anything as petty as money. Maybe a promise to take out Cadmus or her mother?

Actually, thinking about it, I bet Lena isn't aware of who Rhea is, being Rhea is in "human" clothing. Probably Rhea presents what initially appears to be a simple business proposition.