Ask your client to take the following steps
Sign in to QuickBooks Mac as the admin.
Go to File menu and select Export. Select To QuickBooks for Windows.
If you get a prompt to verify before you back up, select Verify.
Name the file and choose the folder to save it in. Then, select Save.
Note: Make sure you don't add special characters in the file name.
Enter and confirm your password, then select OK. Select OK again.
Send the file to your end and try to restore it on your QB Desktop 2019 (Windows).
If you you encounter error, ask your client to download/open the sample file and repeat the steps. Did you encounter the same error?