First look at where the toes are pointing. Toes straight forward often result in knees caving in. This is common to see in beginners, especially when using a narrow stance. Just pointing the toes out a tiny bit and perhaps widen the stance a few cm goes a long way.
If the above isn’t enough, tell the lifter to "screw their feet outward" before descending. The feet won't physically move but it creates the proper tension. Once ascending focus on pushing the feet out (and absolutely not down) by "spreading the floor" so to speak.
If this doesn't work the lifter will either have to narrow or widen their stance. If the lifter has a too narrow stance the hips are likely not doing their job and a too wide stance might be too much for the mobility of the lifter.
First try to tell the lifter to "pull themselves down". This is a hard movement to describe in words but there's an easy exercise to get the feeling for it.
If this doesn't help you could vary the bar- and hand positions. A too low bar placement, or too close hand spacing will cause excessive forward lean coming down.
The first cue to try is to tell the lifter to "drive their back through the bar". That means straight up through the bar, not throw it backwards. This is a powerful cue that often solves the problem.
If the lifter squats closer to the so called "Olympic style" (not that such a style exists but it's a phrase many are familiar with so we'll use it) which means bar placement high, feet fairly close, and not sitting back very much, then the issue is in most cases lack of leg strength.
Can be too close hand spacing and too low bar placement for the lifter's mobility. Correct accordingly.
Sometimes a lack of back strength or insufficient tension in the back. Make sure the entire upper back is tightened, which means shoulder blades down and back "in the back pockets". It can help to pull on the bar and "try to bend it".
First assure that the initial rounding isn't happening in the upper back. Assuming it doesn't...
While it can be a mobility issue (especially if the lifter squats behind their legs rather then between them) it's often the case of improper tension, particular in the pelvic are and core. Make sure that the lifter tense the core properly and understand correct power breathing.
If that still isn't enough tell the lifter to "screw their feet outward" before descending. The feet won't physically move but it creates the proper tension.
If the issue still persists change the foot placement.
If this still doesn't solve it then a period of squatting with light (less than 70%) to moderate (less than 85%) weights just to the point where the issue arise will build up mobility.
If a lifter cuts depth due to fear the best way to overcome it is by lowering the weights and gradually increase as the lifter becomes more comfortable. Pause squats is also a valid variation to use, as are Front squats.
If it’s a mobility issue then a fast way is to practice squatting more regularly, even outside of the gym. Have the lifter sit in a full squat position (without weight, perhaps aside from something very light to counter balance) daily, perhaps while watching TV or waiting for coffee to finish. Practicing Horse stance can also help.
Usually a case of being plain too heavy but it can be a technical issue. Two common technical mistakes is not staying tight throughout the lift and unintentionally pausing in the bottom. In the first case it’s likely a good idea to slow down the descent as a too rapid dropping can cause one to lose tightness.
If it’s desirable to build more strength out of the hole one can use Pause squats, Front squats, and even very low Pin squats.
This is the most common sticking point and is usually mostly a case of “get stronger”. It can also be because a lack of weak hip extension. In that Box squats can help a lot.
This is rather uncommon but Box squats can help in this case.