Edward,
Notice what the woman saw coming up, it was "eloheem", gods. According to the OT and the NT, the gods were demons.
17 They sacrificed to demons, not to God, To gods they did not know, To new gods, new arrivals That your fathers did not fear. (Deu 32:17 NKJ)
20 Rather, that the things which the Gentiles sacrifice they sacrifice to demons and not to God, and I do not want you to have fellowship with demons. (1Co 10:20 NKJ)
Personally, I am not convinced by this argument. Primarily because it relies on the perspective of the medium.
The author of this book however, designates the being as Samuel.
Then
Samuel said to Saul, “Why have you disturbed me by bringing me up?” Saul answered, “I am in great distress, for the Philistines are warring against me, and God has turned away from me and answers me no more, either by prophets or by dreams. Therefore I have summoned you to tell me what I shall do.” 1 Samuel 28:15 (ESV)
Saul even knew that it was Samuel.
He said to her, “What is his appearance?” And she said, “An old man is coming up, and he is wrapped in a robe.”
And Saul knew that it was Samuel, and he bowed with his face to the ground and paid homage. 1 Samuel 28:14 (ESV)
If this isn't evidence enough, what Samuel says makes it quite clear that he is not a demonic being.
And Samuel said, “Why then do you ask me, since the LORD has turned from you and become your enemy? The LORD has done to you as he spoke by me, for the LORD has torn the kingdom out of your hand and given it to your neighbor, David. Because you did not obey the voice of the LORD and did not carry out his fierce wrath against Amalek, therefore the LORD has done this thing to you this day. Moreover, the LORD will give Israel also with you into the hand of the Philistines, and tomorrow you and your sons shall be with me. The LORD will give the army of Israel also into the hand of the Philistines.” 1 Samuel 28:16-19 (ESV)
Samuel recounts precisely what he predicted would happen on account of Saul's disobedience, and even further, prophecies that Yahweh would give the army of Israel into the hand of the Philistines and that Saul and his sons would be dead on the following day.
This came to pass in this verse:
Thus
Saul died, and his three sons, and his armor-bearer, and all his men, on the same day together. 1 Samuel 31:6 (ESV)
I don't believe that demons have the power to understand what will happen in the future, nor do they have a connection to God's sovereign plan to understand such. The words spoken by Samuel, are such that you would expect from a prophet of God.