Crosshairs moving all over the place is probably parallax. You either need to set the adjustable objective to the correct yardage or realize that if the scope doesn't have an adjustable objective the parallax will only be perfect at one distance.
Also, if the scope isn't on the rifle in the spot where your father wants it, his eye won't be at the right distance and he'll never get a solid picture. Have him shoulder the rifle with the scope loose enough in the rings to slide and move it to where he wants it.
On the matter of adjustments, this should obviously be straight-forward. 6" off at 100 yards would be 24 clicks on a typical 1/4" adjustment scope. I have found, though, that low-end scopes do not typically adjust the actual amount that's listed. In other words, a click that should be 1/4 might be 1/8 or 3/8, even 1/2.
Using such scopes, I always do adjustments at half the number I expect until I get an idea of how far a click really moves it. It should be easy. If you need 24 clicks, do 12 and shoot again. If it moves more or less than you expected, figure out the true per click adjustment and go from there. Make sure you're shooting enough shots between adjustments so you can get a good average too. Making adjustments on one shot will have you chasing all day.