A non-expert's view here (albeit one who teaches spin and is an avid cyclist): make sure his saddle is adjusted properly, for height, tilt, and fore/aft. At the bottom of the pedal stroke, he should only have 5-15 degrees of flexion at the right height, and with his pedals parallel to the ground, his tibial tuberosity should be directly over the pedal spindle. If he is too low, this gives him the opportunity to allow his knees to wander. At the right saddle height, you can help minimize this. FWIW, most people set the saddle too low, and this gives them the opportunity for knee wobble if they have the limitation Julie talked about above. Take away that opportunity and it will help stop enabling the wobble.
A second part of the equation could be the type of surgery he had and how it affects his alignment with the pedals. For example, in my case, I am missing my lateral meniscii in both legs, which throws off my entire leg alignment for each one. I correct somewhat with custom orthotics in my bike shoes, and also with different pedal and cleat combinations. A knowledgeable bike coach or very good local bike shop can get more information about that to you.
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