It's how we do it actually (Brazilian guy here). It would be only wrong, depending on what you want to mean, if you put the "ninguém/nada/nem" before the "não", for example: "Ninguém não viu ele.". That is incorrect - if you want to say that nobody saw him. "Ninguém viu ele" is the correct form.
In the fast version, which I think is a feature of duo, it sounds like she is purposefully connecting não and viu together, BOTH at the end of não AND at the beginning of viu, which makes the fast version sound like ouviu, which is also a word that would work in this example, so, I had to listen to the slow version to get it.