In her final fight against Molly Weasley, Bellatrix taunts her saying “What will happen to your children when I’ve killed you? When Mummy’s gone the same way as Freddie?”

How on earth did she know that it was Fred who died?

George still looked like Harry when he lost his ear and they have been hiding since.

Bellatrix was nowhere near when Fred died.

Or are the death eaters running a rumour mill?