Praise be to Allah.
Firstly: her Hajj is valid, in sha Allah, but her travelling without a mahram is something that is forbidden, as it disobeys the words of the Messenger (peace and blessings of Allah be upon him): “A woman should not travel except with a mahram.” This lady travelled without a mahram. If she was unaware of this ruling, then I hope that she will be excused, and she has to seek the forgiveness of Allah. If she was aware of it, then she has to repent and seek the forgiveness of Allah.
But there is another question to be asked, which is: does this Hajj count as the obligatory Hajj required in Islam?
If she had reached the age of puberty when she did that Hajj, then it does count as the obligatory Hajj required in Islam, even if she went without a mahram. If she had not yet reached the age of puberty, then she has to go for Hajj again, and her first Hajj will count as naafilah (supererogatory). What is meant by puberty is the appearance of one of the signs of puberty, such as the onset of menstruation, the growth of body hair (in the armpits and pubic area), or “wet dreams” (following which one detects the emission of fluid from the private parts). It is most likely that a woman of fourteen years of age had reached puberty.
And Allah knows best.