Betta Fish: Fixing a tank

So my neighbor is moving and they said they’re gonna flush the fish if no one takes him. I currently have a 30 gallon tank with an angel fish, I know they’re not good tank mates, but I do have some aquarium experience. I volunteered to take him in. He’s in a 5 gallon fully planted tank, I haven’t worked with anything but gravel and one live plant. I’m picking him up tonight so I don’t know the water parameters, I just know the tank is “dirty”. I do have some test strips at home I can use. Could I gravel vacuum the bottom and replace half the water? Should I take some plants out? And if so, how do I determine if the plants are alive and healthy. She said she also has a carbon filter to get rid of the ‘tannin’ coloring in the tank and a typical filter.

Tl;dr: I’m rescuing a fish and want to know the best way to clean and help his tank.