Get the Gebhard or the Rubinstein The Art of Pedaling.
Summary: stop using it as a switch and use it like a clutch.
If the problem is blurring and gaps, step one is that he SHOULD use it like a switch. The first step is to get used to hearing blurring as the new sound enter, then switch to completely dry clarity and finally listen for slight blossoming of the sound as the foot goes down again. Every step should be based on hearing a cue and then taking that as permission to go to the next step. Unless you've heard the desired cue, the next step should not be allowed to commence. You need a feedback loop that is founded upon checking each result before the next step is even on the radar.
With that foundation, there's plenty of room to start exploring the pedal like a clutch. I rarely change the pedal without some kind of fluttering after the main change. But if blurring and gaps are common due to improper pedal timing, stage one is to learn to time it flawlessly as a simple on/off switch. Complexity should only begin after the basic coordination is flawless.
PS. The above exercise is all that is needed for basic pedal skills. Play any series of chords and look for every step on every new harmorny- blurring, complete clarity and finally a slight addition of resonance to a previously clean sound. The blurring may be milliseconds, but it's an essential step of getting to grips with pedalling. If there's no blur, it's usually because the foot let go prematurely. Blurring followed by clarity is what tells you the basic premise is working.
Topic: Anyone got some pedaling exercises I can use?