Once your client's QuickBooks Online account is deactivated, you're given a year of read-only access to get all the data you need from your company. Just login to your client's account and export their reports and lists. I can guide you on how to do it.
Go to the Gear icon ⚙.
Choose Export Data.
On the Reports tab, set the date range.
Add or remove items from the Reports and Lists tabs by toggling the slider.