Obese friend has a veryyyy smelly problem! HELP?

Ok so I have this fat friend. Really F A T! And she has a strong musky scent. I sit next to her at work and everyyyytime she stands up ... OMG... I feel like gasping for air! The smell coming from her bum is overpowering! The heat coming from her bum will make your eyebrows fizz off! It smells like B.O, poo, sweat, vomit, garbage... and more poo!!! And its like she can't smell that sh*t!!! Once...... a tear rolled down my face when she stood up! How do I tell her that her bum is grotesquely smelly without being mean????? And please don't tell me I'm mean! YOU HAVEN'T SUFFERED LIKE WE HAVE!!! *sigh* thanks.