She will report this in 2020 unless she operates a business that uses accrual-based accounting.  This is rarely used for individuals doing contract jobs - so it is not likely that it applies in her situation.  

Thanks for your answer. 

What I understand that her earing should be reported in 2020 tax (filling in 2021). Am I right? Please confirm. 

Thank you very much for further clarification.

You are welcome!