If kefir is fermented correctly, meaning at the correct temperature and long enough, the lactose is absorbed by the bacteria during the process, and there’s a very limited amount of lactose left in the end product. The beneficial bacteria cannot develop and grow if they have no food source. Keep in mind that yu can also make water kefir.
Hopefully the commercial Greek yogurt is fermented long enough to do the same. But this is why a high quality brand should be purchased. Also start very slowing with both kefir and Greek yogurt