Well, for one, I approach critiquing/reviewing beginners differently than people who already have some experience drawing or are really passionate, are drawing a lot, and really want to get better and everything they post in their sketchbooks screams that they are putting in loads of effort.
I like to think that there's a two-or-so month beginner period. It's a period where it's alright if you're doing things completely ass backwards or making completely blatant mistakes. But it's also important that the artist tries to learn from video tutorials, and learns how to learn to draw. As well as draw a lot, and look at a lot of art. Encouragement is also important, and small recommendations to the artist on how they can improve is also beneficial. It's really like the person's first taste of drawing. It's good if they learn to have fun drawing in this phase. Stay away from excessive nitpicking.
The reason I think this beginner phase is important is because knowledge you can find in books becomes more valuable to you if you go through this phase. At least that's what I think. You will appreciate the knowledge and insight you can learn from reading great books because they are solutions to the mistakes you made in the past.
One of the best things you can do for another artist is to tell them that the amount of effort they're putting in is dropping.
No matter what level the person is at, if you notice he/she is putting in less effort it is an absolutely powerful statement to tell them that you see this in their work. And if they're no longer innovating their work, or finding new ways to do things, tell them to try to do this. It doesn't even matter if you are able to recommend ways to do new things for them, just tell them to do it. And it doesn't even need to be technical-related. It can be subject matter related. Encouragement is powerful; we all need a kick in the ass every once in a while.
Try to say both good and bad things if you're reviewing somebody(if you decide to say anything negative at all). It really helps to point out both the good and bad things in somebody else's work. They will appreciate the review from you more.
Give technical critique if you're capable to fill the hole in the recipients knowledge. For example, telling somebody why the deltoids looks wrong or what the proportions really should look like of a human figure or a way they can fix all this. If you can't explain it, just say that "the deltoids look wrong or weird." Chances are they will be able to sufficiently find out why and they will be able to fix it on their own.