To a couch potato, any routine is better than no routine. So, if his goal is to look good sitting on the couch, the muscle imbalances might not be a problem. But if he wants to get off the couch - even for casual physical activities - problems will arise.
As noted in hoosegow's article, serious sports trainers no longer consider the leg press as a substitute for the squat. Leg press trained athletes don't have the lower back strength needed to transfer the power generated by their lower bodies to their upper bodies in their athletic movements. Doing squats and deadlifts in a routine avoids the lagging lower back problem.
Olympic trainers worried about the load that the back squat places on the spine switched to a version of the step up as a work around.
Frankly, a commentor who is making progress squatting worrying about getting injured is a new one on me. As Matt and Robertscott said, you don't have to set PRs in the squat to benefit from the exercise.
And, one more quote from hoosegow's article:
Piling 45s on a leg press doesn't make it equal to a squatWorld renowned personal trainer Brian Dobson, owner of Metroflex gym, says, “My daughter can leg press 800 pounds, yet she struggles to squat 115.”