Alright, so my friend is working on a ribbed scarf, but no matter what he does, it refuses to rib. Any tips, and/or common problems you could alert us to? Thanks a bunch. 
Ribbing help
What do you mean? It won’t stretch? Or the ribbing is invisible when you stretch it? Is “your friend” alternating knitting and purling stitches? That would be the trick.
To rib you need to knit the knit stitches and purl the purl stitches. So if you have an even number of stitches it would be like this:
row 1- k2 p2 k2 p2
row 2- k2 p2 k2 p2
I’m making a hat and I cast on 100 stitches. I put a marker at the join and just k2 p2 all the way around.
HTH 
Ah… i think i see what it is. When you turn the work over, you have to remember you’re doing the opposite. The stitches you knit on the one side, you have to purl on the opposite side. And vice-versa. Do you have an even number of ribs, so you start each row with a knit rib and end with a purl rib?
If you have an odd number of ribs, you must start and finish with knit ribs on one side, and start and finish with perl ribs on the other side. Amy has a video about ribbing, which tells you how to recognize what kind of stitch was done on the previous row.
Okay, I should mention that I am knitting the hat in the round. You still knit the knits and purl the purls, but the stitches in the round are always on the front side.
When ribbing flat you still knit the knits and purl the purls. You either have to remember what the last set was OR learn what purl and knit stitches look like. I vote for learning what stitches look like. It has helped me tremendously!
I have an even number of ribs on my hat in the round. since i have 100 co stitches I guess it would be 50.
Also, if you work in k2p2 rib, you need to have a multiple of 4; for kp rib, a multiple of 2.
Thank you all so much! 
It turns out he had forgotten to change his knit stitches to purls when he flipped the scarf over. Thanks for being so prompt and generous in your responses!