In answering this question, I don't mean in some existential sense, for we are all prisoners who are trapped within bioelectromechanical meat bags and can only perceive the world from limited perspectives. No, I mean in a physical sense; one that can be empirically measured and yes, it can be.
Two score, ten and three and a bit years ago, the president of a certain nation committed his nation to the goal of spending an insane amount of money to send some clown to the Moon as a distraction to stop the president of another nation spending an equally insane amount of money to send rockets for the purpose of blowing people to pieces with nuclear weapons. That president would suffer a sudden existence failure when his head failed to provide adequate protection and stopping power to a bullet which only minutes before was hanging out in a dark place with a bunch of its mates and going about its dull bullet life. The bullet after passing through the president's head; thus fulfilling its life's work, was never heard from again. The president's story, has been heard many times over.
For the next decade, that nation did spend an insane amount of money and sent thirty clowns to the Moon; twelve of whom would actually get to kick the dust upon its surface.
During each of the six successful missions which did deposit bozos to the Moon's surface and return them safely, two of them would go down in the Lunar Module while the third would remain in orbit around the moon. It is in that period when the bozo in the Command Module was on the far side of the moon that he would have been the loneliest person ever. The two clowns who were merrily dancing upon the moon's surface, looking at rocks, setting up experiments, looking at more rocks, playing golf, looking at more rocks, driving a car so that they could look at different rocks, before stealing rocks and leaving, would have always been no more than a couple of hundred yards away from each other but the one bozo on the far side of the moon would have had no other person within 2240 miles of him; the nearest two people would have been down on the moon's surface.
Sheer logic tells me that the loneliest person of all time must have been either on one of those six missions or on the full dress rehersal in which they didn't land on the moon but I don't know which one.
The loneliest woman in history is far harder to ascertain. The most obvious candidate that I can think of is Valentina Tereshkova because she was the only person in space at the time. The problem that I have in determining a definitive answer though is that even if she was hanging over the a point above the Pacific or Atlantic Ocean, she was still only about a few hundred miles away from someone on the Earth's surface. It is entirely possible that someone out in the desert, such as a radio operator in the middle of the Simpson Desert might have been physically further away from another human than Valentina Tereshkova was but I have no reliable method of finding out who. A few hundred miles is an order of magnitude less than two thousand miles.
The loneliest man in history must have been one of John Young, Michael Collins, Richard Gordon Jr, Stuart Roosa, Alfred Worden, Thomas Mattingly or Ronald Evans but I don't know which one. I do know that there is a definitive answer though and that it's one of seven.