I have a client who's customer pays 30 days after statement date. How do I setup this customer term in Quickbooks?

Let me provide you with some information about payment terms, Elene.

Invoice payment terms are an agreement between you and your customers on when payments are due. In QuickBooks Online (QBO), there are available terms that you can choose. You'll want to select the term that you have agreed upon with your customer. I've added  a screenshot for your visual guide: 

For more information, I've added this article to your guide. This is a US article but it works similarly: How To Adjust Invoice Payment Terms In QuickBooks Online.

Additionally, I've added this article to show how you can record invoice payments: Record Invoice Payments In QBO.

Let me know if you have any questions about payment terms. I'll always be right here to help.