That won't help. If there's a crack or a break in the seam of the two halves of the headlight, the moisture will just come back the next time he washes his car or when it rains.
Take your headlight off, and bake it in the oven for 10 minutes. (Pre-heat to 350 degrees, then turn it off... and place it in the oven for 10 minutes with the oven off). When you take it out, take apart the two halves and put them back together. Use some sort of clamp to press them together tightly, and the glue will harden in about 30 minutes. Then replace your headlight.