She doesn't turn the hourglass over. She breaks the hourglass, and gives the old man the sand, so that he can get back to the group with the information of where the fortress will rise next (it both keeps him alive and gets him past the spider). With the hourglass broken, she has no further control over the guardian, and it eats her. When the old man runs out of sand, he dies as well, after passing the information on. The fact that his remaining life would be tied to the sands is the price he pays for the widow to tell him what he needs to know.
It looks like the sands are some sort of an artifact that controls the spider, but once you begin to use them, your life is tied to them. The widow had survived this long by having them in the hourglass, but once they're outside of it, they are expended when they fall? It's not well-explained. Basically, Krull is really big on having heroes have to die every step of the way.