Create the email that it was sent to if you can. i.e.
[email protected]
Add it to Paypal
Get customer to send you $1 to the email address again
Hopefully it will prompt the rest of the funds to be sent through.
If not, contact Paypal and explain the situation with a copy of the receipt to where the cash should have gone. They'll fix it up for you, most likely. Failing this, the customer can do a chargeback and it might be instant if the email address doesn't exist.