If I have a boarding pass (Air Asia), is there any fare or is it free to use K5 line bus in HK?

No, you cannot take the free shuttle bus if just showing the boarding pass. Only Airport Express passengers can enjoy this service.
Generally speaking, passengers are required to show proof of eligibility including AE train tickets (Single Journey, Same Day Return, Round Trip, AE Travel Pass), Airline ticket / boarding pass, Airport Staff Octopus Card, AsiaWorld-Expo entry pass or event ticket.