If the are several balls of yarn your wife might look at the beginning of each and see which would be the best to start the body with (likely still pulling some out and cutting the yarn to start in the solid colour but cutting less off, if that makes sense? The cut off yarn can be used for seaming if there is any).
Also, planning ahead on the yarn balls can help. For example finding the same starting point for both sleeves on two different balls so that the sleeves turn out the same… unless she fancies an asymmetrical look to the sweater which is often beautiful too. Once identified, the 2 yarn balls can be labelled with a stitch marker as a reminder to save for sleeves.
And if a new yarn ball is needed mid front or mid back, to pull out the new yarn ball to get to the same point in the colour variation as the old yarn ball where it runs out, allowing a tail in each yarn end to weave in later.