Unfortunately he didn't know that and being from the UK did not think he would have any issues with immigration in any country in the world, least of all the US

Unfortunately he didn't know that and being from the UK did not think he would have any issues with immigration in any country in the world, least of all the US! I thought transit was different from entering a country for a stay, for example I know that people from most countries need a visa to visit India, but if you are transiting through then you are just waved through. Same with most other countries as far as I am aware... Regardless, he is now not keen on visiting US in any circumstance and would rather pay more/take a longer flight than transit through the US.