As a customer, it is understandable that they sometimes want to do a quick peruse through the ticket and not log in to the portal. In this scenario, the best recommendation would be to use a "public ticket URL" which leads to the ticket and does not require the customer to sign in.

This has a placeholder which when included in the description of the ticket will ensure that the customer can view the ticket status without logging into the portal upon clicking this URL. 

To have the Public Ticket URL available in all replies, please navigate to Admin > Workflows > Email Notifications > Templates > Agent Reply Template -> insert placeholder and include the Public Ticket URL placeholder (please find this under the tickets section within the "insert placeholder" window).