Edit: Just asked my sister. She's in vet school. She said:

"The stumbling symptoms sound like it could be an inner ear problem which could be affecting the cat's balance. Other than that without seeing the cat myself I'd have to say it could be seizures. I'm tempted to think it's not a tumor in the brain because that would typically cause more of a constant problem and would make the cat a lot less frolicsome all of the time. You should tell your friend that he should take his kitty to a vet, as I'm not yet qualified and I haven't even seen the cat myself. I hope that's at least somewhat helpful".

Well that's actually really inconclusive, sorry. Have someone qualified look at your cat Chojin, it could be really mild but you have no way of knowing until you do. Good luck.

I had a cat that had an inner ear problem and it sounds just like what your cat is doing. It wasn't too expensive and they gave me drops to put in his ear. It cleared up pretty quick, even if my arms didn't from all the clawing.

That's a reallt cute cat, btw.