Keep a deodorant aerosol spray on your desk. Encourage your colleagues to do the same. Then, every so often say in a loud voice "Phew, isn't it hot? I'm sweating like a pig. Excuse me while I freshen up, otherwise I'll be stinking everybody out. Anyone else want to borrow this?"
Then undo your top two blouse buttons and puff it under your armpits.
That brings the sweating out into the open, acknowledges that it can be a problem for everybody in this heat and may drop a hint to her without causing her to lose face. If she doesn't get the message after two or three times of this happening, you can pass her the can saying, "Here, think you might need it too."
If this still doesn't work, maybe your boss might need to have a quiet word in her ear, but try not to humiliate her. She may not realise she has a problem, or may just have the type of glands which cause her to sweat more than average.