You have to choose to affect either 2019 or 2020. Since 2019 book was closed and tax return was filed, it is more simple to apply this payment in 2020. Are you on accrued or cash basis? If your book is accrued, remember tax return is cash basis. If you receive more payments in 2020, you pay more taxes in 2020. 

For long run, you should have your friend to create invoices and/or sales receipts, not JEs, in QBO. 

Hope it helps. Please let me know if any questions

Anthony