Poor hygiene is a classic symptom of depression, it's a form of apathy. If you don't really care about anything, or have low self esteem, your not going to care what others think about you.
Screaming at someone or embarrassing them about it won't do anything but cause things to be even more awkward or hostile.
Not sure if I buy the whole not drying off or using enough soap stuff either. I don't do it myself but I've heard of people who won't use soap because of the chemicals (hardcore primalists or vegans), so they just wash well and supposedly smell fine.
Now, showering with really hot water, or over-drying doesn't do any good because your almost immediately sweating after you get out of the shower. The hot water or rough rubbing on the skin opens up your pores and makes it easier to sweat.
The two possible solutions are either, 1) shower with cooler water and don't dry off so much that the towel feels like sandpaper, or 2) They need to find something that drags them out of their apathy, and makes them feel like they have some purpose. After that it feels good to take care of yourself. Getting into a relationship or learning a new skill are two things that accomplish that.