With a beginner student, I would initially just tell him or her that notes from the G Major Scale will sound in tune if he or she is soloing or playing a melody over the D7 Chord. If the student asks why? I would tell him or her because all of the notes that make up the D7 chord are contained in the G Major scale. That is about as simple as you can put it. The raising and lowering stuff will only confuse your student. I would focus on having that student learn the Major Scale through the circle of 5ths before introducing modes and all sorts of altered forms. It's like, you can't start building a house before you pour the foundation.