Ideally, he wants an ambidextrous friendly light. Many of the side mounted tube lights, such as the Surefire G2, are hard to use with the other hand because of the way we hold the forestock of the rifle. That is why I put a Streamlight with rocker switch on top of my rail. Both hands can reach it and activate it easily.
Tell him not to bother with the gimmicks. He doesn’t need hi/low beams or strobe effect. No integrated laser either. That should be a separate unit.
Mount the light as far forward as is comfortable for both hands to reach. If he is using a front sight that is mounted on the upper rail, he can put that behind the light. The difference in distance from the rear sight won’t make a difference that he will notice.
Try to get a light that uses AA batteries. Those CR123 batteries are nice, but kind of pricey. Change the battery at least every year (CR123) or six months (AA). You can buy the CR123 in bulk for about $25 and they store well, but AAs are cheaper and everywhere.