I think it has to do with your dvd drive and his dvd drive ...
My xbox reads all dvd's and my friend does not ... His only reads DVD-R ... Mine does both ...
You should find out what drive he has ...
on
www.xbox-scene.com you can find out wich xbox got which drive ...
Goodluck!
And I always burn at 4 speed! ... It is the media that does the trick ... ;-)