Answer: Did anyone witness Lot's wife turning into a pillar of salt? The short answer to this question is that many times we must have faith that a biblical event actually occurred. No human, after all, witnessed God creating the universe or him making Adam. Countless other events recorded in the Bible must ultimately be accepted as valid and true based on faith.
It should be noted Lot's mate did not miraculously become a mineral for merely taking a quick glance behind her to see the destruction of her home in Sodom. The English word "look" used to describe her action in Genesis 19:17 and 26, comes from the Hebrew nabat (Strong's Concordance #H5027), which means to look intently in a favorable manner.
Lot's wife was punished by being turned into a mineral pillar because she viewed longingly and regrettably at the destruction of the two great cities. Her attitude about Sodom, and her willingness to minimize their gross sins that brought their punishment, may have rubbed off on her daughters.
The attitudes of their mother could have played a part in Lot's daughters willingness to soon do whatever it took to ensure they would have children (see Genesis 19:30 - 38). Her attitude certainly played a large role in her own destruction through salt.
Lot himself, not surprisingly, hesitated as he left Sodom. The two angels who were sent to lead him and his family out of Sodom had to grab their hands and take them quickly out of harm's way. As they dashed out of sin city, the angels told the family, "Run for your lives! Don't look back and don't stop in the valley" (Genesis 19:17).
When the family reached the small town of Zoar, just as the sun rose, "Suddenly the Lord rained burning sulfur on the cities of Sodom and Gomorrah . . ." (Genesis 19:24). It was then that Lot's mate turned to look at what was happening and became a pillar of salt (verse 26).
As a side note, many Bible commentaries say that as she longingly gazed at Sodom she may have turned into salt from all burning or molten sulphur (translated as "brimstone" in the KJV Bible) that fell on her from the sky. They state that what fell on her may have crystallized her into a pillar.
A possible witness
Lot likely witnessed the miraculously demise of his wife. It is only natural for the person in the lead to glance back to insure the rest were following him. Unfortunately, this loss was not the end of his trials and troubles. Right after God's judgment of his mate, his two daughters carried out a misguided plan they thought would insure the continuation of the human race (Genesis 19:30 - 38).
The turning of someone into a pillar of salt was such a significant event that Jesus used it as an example of what not to do just before his Second Coming back to earth.
And it was the same way in the days of Lot: they were eating, they were drinking . . . But on the day that Lot went out from Sodom, it rained fire and sulphur from heaven and destroyed them all . . . Remember Lot's wife (Luke 17:28 - 29, 32, HBFV).