The encounter happened as Robert and his fellow climbers headed through rainforest en route to tackling the 19,336 feet-high Kilimanjaro, in Tanzania.
"We passed him on the way up," said Robert, a 64-year-old Glasgow taxi driver.
"He was a young boy and he had a wee chameleon with him, and when we came back down we saw him again, still trying to sell us one. He must have had a chameleon farm."
Robert, of Bishopbriggs, has been driving taxis in Glasgow for 35 years.
He made the trip to east Africa to raise funds for the British Heart Foundation, and in memory of his brothers Iain and Alex, also taxi drivers.
One died of a heart attack and the other of a brain haemorrhage.
Robert says: "Any reasonably fit person can climb Kilimanjaro," but he himself is pretty fit.
He has, in his time, climbed all of Scotland's 282 Munros. And this summer, before flying to Tanzania, he climbed 25 Corbetts - Scottish mountains between 2500 and 3000 feet.
But as his highest previous climb had been the 4409ft Ben Nevis, he knew he faced a real challenge in Kilimanjaro.
"It was an amazing experience," he said.
"The first place you get to is Mandara, at 9000 feet, and from there you go over moorland. It has heather, which reminded me of Scotland - but whereas ours is about two feet high, over there it's about 15 feet.
"The next stop is Horombo, at 12,200 feet. You spend two days there in order to acclimatise, before heading out again.
"You soon come to a barren Alpine desert."
On the fourth day Robert's party headed for the camp at Kibo, at 16,300ft. From there to the top, it was roughly 3000 ft.
"That last stretch was probably just like climbing a Munro, but with one difference," says Robert.
"Because of the high altitude, you are short of breath, and some climbers can get bad headaches.
"I'd met another Scottish guy, Brian Ross, who was our official photographer, and we were OK, but there were some German people who complained of headaches."
Brian, it turned out, had climbed Kilimanjaro 40 years earlier. On the final ascent, the climbers started out at midnight.
"The ground has frozen by then - there's a lot of scree on the path, but it's easier to walk on when it's frozen," said Robert. "The guides kept telling us to walk slowly, not to rush.
"One of the guides, Abel, made it look easy, though. He was walking up the path with his hands in his pockets. In time we got onto the rim of Kilimanjaro's crater at an area known as Gillman's Point, and just as we did, it started snowing.
"We walked around the crater rim for a mile and a half until we got to the peak at Uhuru.
"There were people converging on the peak from all the different routes up the mountain, and we didn't hang around long, not least because the oxygen level is only half of what it was lower down. We got our photograph taken and we started the long journey back down again."
Robert hopes to raise £500 for the British Heart Foundation.
l Robert's Just Giving page can be found at www.justgiving.com/Robert-McGill