If you have a hub which can accept an 8 speed cassette, it probabaly won't take a 6. I don't think there is one available which would fit the splines, not sure, but it wouldn't be the correct width.
If he MUST have index shifting, you could look into the cost of modern downtube shifters for a modern cassette. I believe you can still get them.
I hope the frame was steel because you're not supposed to change an aluminum frame to take a wider, modern mountain hub (deore), but people do it anyway. It can crack.