still wouldn't fit if he is having issues with the current set up since width is in inches and offset is in mm
He would rub more with the 17x9
Not necessarily.
If he chose the proper offset to bring the rim in more, and then put the same tires he has now on the 9" rim, it would provide him a slight less beef, more stretch, and could theoretically fit them easier.
Modifications? I've forgotten more than I remember. Click here for the list.