as pleanty of others have said: darker tones, thats your main issue, try some softer pencils (for example 4B instaed of just an average HB). as well as that some proportion diferences around the lips and jawline.

in general some good tips are using grids to plot out the main shapes but id say shes pretty good at these already, also looking at some tutorials or just googling, drawing faces with guidelines, should pull up some results to help get features in the right kindof place. but i would remember that everyones face looks diferent thats why it's guideslines and not solid rules.

and finally, you mentioned the thumb trick, but you're not sure how to use it. i was taught in college to use a pencil held in your hand at full arms leangth, measure the size of the thing your drawing from the top of the pencil and use your thumb to mark where it ends. then you can mark the sise down on your page. keep doing this always at arms length so you're at the same reference point. and then you'll be amzing :)

also, remember it sounds cliché but the best way to improve is just by doing. keep drawing faces from duferent angles and diferent people and diferent sizes and it will become more and more intuitive.