I asked a question regarding this story on Yahoo Answers. This is what happens in Judges, chapter 11:
Judges 11:30-31 And Jephthah made a vow to the Lord, and said, "If thou wilt give the Ammonites into my hand, then whoever comes forth from the doors of my house to meet me, when I return victorious from the Ammonites, shall be the Lord's, and I will offer him up for a burnt offering".
So, Jephthah defeated the Ammonites and his daughter is the first to come out of the house when he returns. (Judges 11:32-34)
Judges 11:39 And at the end of two months, she returned to her father, who did with her according to his vow which he had made.
I asked the Christians why they thought God would allow a girl to be sacrificed when they make such a big deal out of God stopping Abraham from sacrificing Isaac.
Only one user gave a real response and this is what he/she said: "There's one key difference between this story and the one about Abraham and Isaac. God ASKED Abraham to sacrifice his son to prove Abraham's faith. Jephthah was just stupid. He was cocky and needed to suffer the consequences of his pride."
So, apparently, some Christians believe God thinks it is moral to allow an innocent person to die because someone else has made an ill-conceived vow to Him.