After what he had spoken to her, she saw that He was not your average person.  Scripturally, a prophet is just someone who speaks forth a revelation from God, but there was a line of thinking at that time that a prophet could have special insight into the hearts of men.

Luke 7:39  When the Pharisee who had invited him saw this, he said to himself, “If this man were a prophet, he would know who is touching him and what kind of woman she is—that she is a sinner.”

Since the Samaritans only recognized the Pentateuch (the first 5 books of the Old Testament) as holy writ, they recognized no prophet after Moses save the one foretold in Deut 18:18, and they rightly believed this one to be the Messiah.  For her to call Jesus a prophet is for her to move into the realm of considering the possibility of Him being the Messiah.

Deut 18:18  I will raise up for them a prophet like you from among their brothers; I will put my words in his mouth, and he will tell them everything I command him.

Also, for her to call Him a prophet in response to what He has just told her is to acknowledge the truth of His statements toward her.

Her knowledge of Jesus was vastly imperfect, but growing. We could all use a greater revelation of Who Jesus is. We accept Him as Savior and maybe even Lord, but He wants to be so much more.