Problems with Players

So, I play DnD 5E with a few of my friends, as a DM. One of them has all the books, which is problematic. We always do this over skype, BTW. So, whenever I describe a monster, he just flips to the page in the Monster Manual and sees all it's stats. This is a problem, but I don't know how to resolve it. If it helps at all, the person's a Level 7 Tiefling Warlock with a pact with the Great Old One, and the Pact of the Chain feature.