She said that she believes we pre-existed in heaven as spirits before being born. Can someone please help me with this?
Gayle:
Welcome to this forum. To answer your question, there is not one single Scripture to back up any such teaching as "we pre-existed in heaven as spirits" before being born. Spirits in Heaven are either angels or "the spirits of just men made perfect" -- those who were justified by faith in Old Testament times and are now in heaven (Heb.12:23). The context will reveal which one it is.
If your mother seriously wants the truth from the Bible, obtain a Strong's Concordance and look through every reference to "spriit" and "spirits". You will not find any teaching regarding spirits of people existing in Heaven before birth and then being sent down to earth to take human form.
This Mormon teaching actually originated in the Kabbalah and Gnosticism, where they teach that spirits originate in God and then evolve back into God, which is exactly what Mormonism teaches. It also teaches that Christ evolved into God, Adam evolved into God, etc. God the Father is supposed to be Adam, and according to them has a body of "flesh".
There's a good possibility that your mother does not really want the truth, since that would mean she would have to repent of all false teaching, including her beliefs as derived from Mormonism. If your mother is not clearly saved, then she simply needs the Gospel, not every reason why Mormonism is false. When the Gospel is presented from God's Word, the Holy Spirit convicts and convinces sinners of their need for a Saviour. That's probably what she needs before anything else.