Aquariums: What’s wrong with my friends betta fish?

Hi there, my friends betta fish is pretty inactive. He spends most of his time resting on stuff – he will flare at a mirror but doesn’t really swim around or anything like most bettas do. From the top it seems like he has a sort of protrusion on one side of his body. I thought he might just be overweight but it looks worse on one side. The way he swims is really weird – sort of like a semi-sideways shuffle rather than straight forward. His fins are extremely long and heavy but maybe some sort of spinal issue or internal problem? Otherwise he seems pretty healthy. He has been like this ever since she got him. She’s just set up a smaller tank for him so it’s easier to get around. Thanks for any ideas or advice.