My friend stinks, and I'm wondering how to approach this?

Tell him to take some responsibility for himself and run some bath water with lots of soap, water, and dishrags, and bubbles as well. Smelling like that is ridiculous and if he's musty, tell him to get newer deodorant. That is absolutely outrageous that someone goes like that out there smelling for too long and whatever it is, he needs to get out of it right away and stop that bad habit.