The most common cause for failure to lock back on empty in a 1911 is the magazine springs. If he hasn't done so, he should dissemble the magazines and clean the springs and inside the mag tube (make sure the top of the follower is NOT lubricated). If the springs are clean, he should try replacing the springs (even then, mags eventually wear out).
The easiest thing to check, however, is the grip. As fasbolt has indicated, a thumb on the slide stop lever when shooting (thumbs forward grip) can cause this. It doesn't happen with all 1911s. For example, I have several 1911s but only one where I have to really watch this.
Also, take the slide off and look to make sure the lip of the magazine follower is actually engaging the follower on the inside of the frame (it should "just" catch it). A mag might begin by catching it but then, after a bit of wear, might miss it.
Jim's Rules of Carry: 1. Any gun is better than no gun. 2. A gun that is reliable is better than a gun that is not. 3. A hole in the right place is better than a hole in the wrong place. 4. A bigger hole is a better hole.
no guns = might makes right