Yes, except if they are located in the US. From the US, your friend can reply through Rebtel if there is a local number assigned. Otherwise, they can’t reply at all.
Outside the US, your contact can either reply directly by using the regular SMS feature on their phone or by clicking on the Reply-link included in the SMS message they receive.
Remember that you need to have at least one mobile number registered on your account to be able to receive replies.