It sounds like a digestive impaction which has resulted in slow digestive transit, hence the food rotting in her crop. She has been drinking a lot which means she is struggling to eat. The egg must have been irresistible to her and too much to carry in her crop.
The crop needs emptying -syringe and tube is best because turning them nearly upside down (chest down) can result in suffocation. Then she needs to clear the blockage and stay clear of sour crop. This means no food just excess water and the process can take a month, if it is indeed successful.
Impactions can be caused by grass, bits of metal or plastic, fruit seeds like damson or cherries, cigarette ends, big pieces of grit. Worms would be a contributory factor. An X-ray would be useful in these cases because a lot of time and effort can be saved and unnecessary suffering of the hen would be avoided.