The trick may be just to get them moving the disc straight back on the backswing and straight through on the follow through. This is often harder to do than it sounds. Also, it may not be the problem. Plenty of good players use a slightly loopy backswing, but they are able to get their wrist and arm in the right line just before release, usually just through practice. Here we are borrowing a term from aviation to describe one of the angles of release.
Pitch refers to how steeply an aircraft is pointed, rotating about an axis through the wings.
For a disc, it refers to the angle of release where the front edge of the disc is pointed up or down. Basically the pitch helps determine how far the disc travels, and more importantly, along which path. By keeping the front edge well up the disc will travel well into the air, but will probably stall and float down at the end of the flight. Keeping the front edge down results in a low trajectory, relatively fast flight. This means whether the outside edge of the disc is held up or down.