On the home page of this site is an article on things to look for when selecting and fitting a pack.

Note particularly the part that points out that a person's height is often not related to their torso length. Torso length is the key measurement (although not the only one) for fitting a pack.

I hope that Dad can return the pack if he's already bought it!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey