If there is no possible way that poison or anything had gotten into the cage, then I would think it would be fine to feed her off. She may have hemorrhaged, had a uterine rupture, it really could be a number of things. Could have had a brain tumor or any other tumor that wasn't visible from the outside, and she could have died from that. Who knows.

You could always do a necropsy and when you are done, sew her back up with cotton thread and feed her off. It won't hurt the snake whatever you decide to do.

Is the rat in the fridge or the freezer?