This is possible depending on how your Shopify store is set up and is not dependent on ReferralCandy. ReferralCandy does not control the behavior or workflow of the checkout process of any store.
Additionally, ReferralCandy does not require a store’s checkout process to be modified in order to run a referral program. Hence, if you want referred friends to log in (or not) in order to use their offers, change your Shopify store’s setup accordingly.