Best way to deal with funky betta water?

My friend was looking after (or supposed to be) my betta while I was away for 3 months. I got back yesterday and my betta does not look so good, laying on the sand. My friend confesses he never changed the water! What is the best way to go about this? Do I put the fish in a smaller tank and completely clean his tank and refill with all fresh water? (His tank is a 5 gallon Eclipse - so has a bio-wheel - and fish has been in same tank for 2 years, it is heavily live planted and heated to 80 degrees) Or would a complete clean like that be a shock to his system, and should I just change like 20% a day? I really want the fish to live as he was my first. Any advice appreciated.