One thing that often happens is the person's mind wanders to things other than God-stuff. What should you do a bout this?
Some suggest that you just "shut it off." My experience is that most are not able to do this. Trying to shut off the wandering mind can turn into an extended meditation on it.
I suggest that, when this happens, you write it in your journal. Write where your mind is wandering to. You won't need to always do this, but I suggest you try it. You'll find that your mind never wanders arbitrarily. It always wanders to something like a burden. In this way your wandering mind is a baromoter of your spiritual condition. Write the burden down. When you get it on paper it can feel like it's distant from you. (Henri Nouwen suggests this in The Genesee Diary.)
The wandering mind could also be a God-prompting. God could be leading you to pray for someone or something. When this happens bring the burden to God, thinking of 1 Peter 5:17, which instructs us to "cast our burdens on him, for he cares for you."
I have found is that the more a person prays and spends time with God, the less their mind wanders. If someone hasn't prayed for an hour for a long time (or ever), one should expect their mind to be all over the place. The more you do this, the less your mind will wander, because - as a lifestyle - you will live in a place of constant de-burdening.
