Why does she use "bad" here? Is it wrong to use "badly" instead?

Native speaker’s answer
Rebecca
Good question! "Badly" is the standard adjective form, but "bad" is also used the same way, as an adjective, in informal contexts. It has the same meaning and is more casual than "badly." You may find "real bad" is used commonly for extra emphasis. Ex: I hurt my foot really bad. Ex: I hurt my foot really badly. Ex: The fan wanted to buy concert tickets real bad. Ex: The fan wanted to buy concert tickets really badly.