His email address should be in the body. You can use that to reply, just copy and paste the email address.
If you set the from email field to: "<Your Customer>" (without the quotes) it will use the email address they entered as the from address.
Trying to use the actual customer email address in the FROM field can cause deliver ability issues. I updated the email settings so that the reply to will work to reply to the customer email address even when you have a different FROM address.