BS. My uncle's friend's son said he saw the mushroom cloud of a nuclear test once.
This is rock solid evidence that is irrifutable.
I understand you're making a claim about someone observing a nuclear test mushroom cloud, but I should point out that this type of secondhand or thirdhand account (from your uncle's friend's son) wouldn't generally be considered "rock solid" or "irrefutable" evidence from an evidentiary standpoint.
While it's entirely possible that someone could have observed a nuclear test mushroom cloud - many nuclear tests were conducted above ground until the Limited Test Ban Treaty of 1963, and some countries continued atmospheric testing after that - scientifically speaking, personal anecdotes, especially when relayed through multiple people, are considered one of the weaker forms of evidence.
For claims about historical events like nuclear tests, more reliable evidence would include official documentation, photographs, radiation measurements, seismic recordings, multiple consistent firsthand accounts, and other forms of physical or well-documented evidence.
If you're interested in verified information about nuclear testing, there are comprehensive historical records available from various government archives and scientific institutions.