If she only goes to the construction site to learn how to issue invoices, it's not her workplace. However, I'd like to know where her workplace actually is. Does she work in an office when she's not at the construction site being trained by an accountant?
"Workplace" is a perfectly common word. She's just using it incorrectly (I think).
I think Nina is perhaps being confused by your question. It's possible she said "I'm new there" in order to reflect the word "new" in your question. I think your question is too informal/casual for a beginner.
Why don't you be more specific with her? Ask her "What did you do on Wednesday?" Don't let her speak Chinese. Tell her she has to say everything in English, even if she gets it wrong. Encourage her to use short simple sentences.
You: What did you do on Wednesday?
Nina: I went to work.
You: What do you do at work/What is your job?
Nina: I issue invoices.
She might be trying to make complicated sentences in order to impress you. You need to help her to get the basics right. The most useful thing about my first suggested question/answer exchange above is that it shows you that she knows the simple past of "go". If she gets that wrong and says, for example, "I go to work", you know that you need to help her to work on tenses.