how can he recover his payment?
He can't. Bitcoin Cash transactions are not reversible and generally cannot be cancelled.
how can we recover his payment?
Create a Bitcoin Cash wallet using your Bitcoin private keys. Then refund him (less transactions fees, agreed handling fees etc)
Given that this involves putting your private keys into an unfamiliar piece of software, there are security issues (you need to be sure you trust it and obtained it from the right place and know how to use it properly etc) there may be significant man-time costs that some businesses may be reluctant to commit to help a small one-time customer.