Despite never being taught your friend does have talent and obviously spends some time drawing, that being said you did ask for "tips"
In terms of proportions it is great from what I can see (although just a face and a hand) the far left finger being the smallest finger should bea slight smaller thr the other fingers. The shading on the hair could carry around theleft side if the head a little more, merely opinion
If your friend cann draw full body proportions well consistently he should consider drawing a (amateur) manga novel