For haunt 41 (Invisible Traitor) they are allowed 3 dice for their sneak attack, which is a max roll of 6. Can he use the Angel's Feather and choose 8 like the card says, even if the max roll he could have gotten using the dice would be 6?
Yes, you can.
The Angel's Feather reads:
When you attempt a roll of any kind, you can call out a number from 0 to 8. Use that number instead of rolling the dice.
Discard this item after you use it.
The feather explicitly says it can be used for a roll 'of any kind'. That's different from a lot of effects in the game which only apply to skill rolls. The feather can be used for a skill roll, the Mystic Elevator roll, a haunt roll, or anything else. That includes the sneak attack roll that you're interested in.
Of course, some rolls, (Like the Mystic Elevator, for example) are undefined for values that are normally not achievable on the roll. This being Betrayal, the rules don't really define what happens if you choose a value outside of the available options. But the feather specifically says that you can call out a number from 0 to 8 on any roll. So I believe that you can use it as written, even if the specified number would normally be impossible.
I would say that the card tells you what happens when you roll any of those numbers. Since you don't 'roll' any of those numbers, you do none of the indicated events, and hence, nothing happens.