how do you know he shifted it without being in neutral? did you watch him do it?
honestly, I think you are overreacting. very likely any sound you are hearing was either there before, or you are fabricating it in your mind, because you think something is wrong.
is it a manual, or auto? either way, shifting into low will produce a bit of grinding if done incorrectly. speed really being the major factor in how much grinding, and thus any damage being done as a result.
though I suspect you could grind it until there are no teeth left, and it still wouldn't cause a noise when driving...
relax and calm down....