Hi .. If buyer has paid then it should show in PayPal probably as pending eBays hold.
Ask your friend to check and make sure there are no issues on their end. Is PayPal email address the same as ebay shows in the listing. Can click on sell similar , scroll to payment section and check the address and then back out.
If their is a problem and the email in listing can be accessed and added to PayPal , the payment will appear.