Rengar is a very versatile champion. Don't let anyone tell you that you can't build Cinderhulk or Devourer on him. You can easily do it, but you just need to understand how and why you're doing it.
Instead of going into the details, I'll explain why you feel the first one is better. It's an extremely reliable way to play Rengar, since that build will usually net a fairly skilled player a kill when they dive. Devourer is much more of a niche item, and it doesn't see a lot of on Rengar. It's more difficult to use efficiently, and it takes a fair bit of practice to understand the item.
Preference is just preference, though. Your friend may just like Devourer on him better. If you're having trouble understanding why some people are absolute die hards for Devourer, look up the item Feral Flare. I'm not sure if you were playing during this time, but this item is basically a different version of Devourer. It has similar mechanics, and this item was the norm in all levels of play (all the way to the World Championship last year).