I'd think any person breeding cuckoo birds would know they have a white spot on top of the head. It's kind of the defining characteristic of the chicks. But, as you said, they're both nice backyard birds.Cuckoo Marans do look similar to Black Australorps to an untrained eye. That's probably where the mistake accrued, unless he already knew. Anyways, both are good layers.
I'm curious, does she lay a darker colored egg? Marans as a rule do, but the cuckoos don't seem to be as dark as the other varieties.