Yes! They can reply C to confirm, R to reschedule, or X to cancel. Anytime your customer replies to an email or text message (SMS) reminder, you'll receive their reply message via email or SMS.
Your customer does not need to have the GoReminders app installed and they are not forced to reply.
You can also view and respond to your customer replies using the 2-Way SMS/Email Messaging feature found within the Messaging tab in the GoReminders app.
For more information on how to respond to customer replies using the 2-Way SMS/Email Messaging feature, check out this article: Reminder Replies and 2-way Messaging