Hi there Malkim,
It's important that your client's books are set up properly so that the information is correctly accounted for. QuickBooks Desktop is a great tool to be able to view the profit of your business by keeping track of expenses and sales. I can provide some information about this.
If your client is running both of these businesses completely seperately, It's best to use separate company files as this makes sure that assets, sales, expenses don't get mixed up with one another. Here's a link explaining how to Create a QuickBooks Desktop company file so that you can create a new file for their other business.
If you have any other questions, feel free to reach out here.
