The kefir grains will consume the lactose, so kefir has less lactose than milk.
Though that is true, kefir still has 20% to 50% of the lactose of milk. Therefore, kefir is not suited for those who have a lactose intolerance.
Kefir made from water/sugar is fine for people with a lactose intolerance.
Milk kefir can also be made from lactose free milk; and thus, would be lactose free.