Buy him a copy of the Illuminati trilogy. At least that takes the attitude that every conspiracy theory you ever heard is true.
Honestly, there is not a lot of point refuting stuff like this. People love conspiracy theories, and the whole point of them is that they are impossible to disprove, since the goalposts get moved with every refutation.
Grains of truth? The US/UK lend lease agreement of WW2, Brits of a certain age remain a mix of proud and angry that “they repaid the US for the war, no matter how much it hurt” with an implied belief that the UK paid vastly more than they though they owed. Not hard to see how this idea is easily reversed. The Allies imposed war reparations on Germany after WW1, the idea that the UK would require payment from the losing side in a war is also something that could somehow be reversed, and the idea that the winner would pay come about. The UK is a lot smaller than the US, and in truth if you think the US has economic problems, you haven’t looked at the dire situation the UK sees itself in. Every bit of industrial decline you might see in the US, the UK got there first. Indeed some would argue that exporting jobs to Asia was a UK invention. A trillion dollars a year would make an astounding difference to the UK economy, and is larger than the entire tax income that the UK government receives. It would interesting to understand how the money is received or spent. But conspiracy theories can always come up with an answer.
Money sent to the UK Monarchy may be confusing the UK Crown with the royal family, and the civil list. The civil list used to the be the list of people paid to run the country. The list of civil servants. The monarch got the money and then paid them. This was in the days when the UK really was a full monarchy and they ran the place. Long since gone. The Royal family still receives money via the civil list, but only for those members of the family that do work that is related to the country. All the royal ceremonies, upkeep etc. About $20 million a year last I saw. But all other money and responsibility for paying for the running of the civil government has long since been the direct responsibility of the UK government. The vast estates technically owned by the monarchy are for all useful purposes controlled by the UK government, which runs and receives the income from them. These are the Crown Estates. This is why when talking about the UK it can be confusing, the actual people that are the current royal family are not the same as the Crown. The way the law works, is complex, and relies upon many unwritten but well established rules. The system requires a living monarch. But it also requires that that same monarch have no influence on the running of the country at all. An attempt by the monarch to become involved in any political component would render them in an untenable position, and they would be swiftly forced to abdicate. This is true for all the commonwealth countries that also have her as their head of state. In one form or another there is a legal need for a living person to be the ultimate root of power. The counterpoint being that they actually cannot wield any power. They have no effective power, even of veto, and if they were to try to exercise even that, the action would trigger a crisis that would soon result in their removal from power. They would be replaced by the next in line, the country still requiring a monarch. So it is nothing like a republican revolution, simply the removal of a troublesome monarch, after which business proceeds as usual.
I’m not sure that Prince Philip supports the green movement much at all. Charles is more the guy for that. He does things like run some part of his estate on methane, and other token green things. (The Windsor family do still own property, and are wealthy of themselves, but this is separate to the Crown, and is nothing like the same scale.) Charles is of course not the monarch. First in line, but not there yet. It is also important to realise that Prince Philip has no power at all. He is the queen’s husband. That is all.
The ultimate fate of the financial system is anyone’s guess. Wiping out the debt and starting again is an amusing idea. It is based on the idea that debt is inherently a bad thing. But makes some astounding assumptions. Mostly that the leaders have any say in this, and such a thing is possible. Or that it matters. It sort of assumes that the finances of countries work like an individual person’s finances. Which they don’t. Cancelling the debt does require that those that are owed the debt agree. Otherwise the term isn’t cancelling - it is default. Cancelling would only work if everyone owed the others what they were owed. If this was the case no-one would care, and the world system would continue on as it was, recognising that holding bonds on one another’s economies brings stability to the system.