Here's a more intuitive and perhaps scientific explanation of how I think.

The theory of evolution by natural selection is wonderful. Its assumptions provide a context in which to frame problems, evaluate solutions, and construct powerful explanations. In particular, evolution enables us to explain adaptation as a product of nature.

How do trees know to lean toward the sunlight? How do birds know to fly south in the winter? How do spiders know to spin a web? More fundamentally: How do the genomes of trees know to build photosynthetic cells? How do the genomes of birds know to build an internal compass? How do the genomes of spiders know how to make silk thread?

According to the laws of physics, the probability of any such behaviour or structure spontaneously appearing is infinitesimally small. It tempts one to believe that adaptations must not be a product of nature; they must have come from somewhere else. This is the intuition behind William Paley's watchmaker argument.

However, Paley's solution merely shifts the problem back a step. If the spider "knows" to spin a web because it was designed to do so by an artificer, then how does the artificer know to build a spider that "knows" to spin a web? Of course, the traditional answer is that God is the artificer and God just knows everything already. But if that was a satisfactory explanation then so would have been the answer that the adaptions just existed already.

The stock answer 'because God did it and God can do anything' might be true, but it's not an good explanation. Rather, it's the implicit rejection of explanation no less than simply saying that adaptions just exist because they do and that's the end of it.

You may wonder what all this has to do with the Münchhausen Trilemma. Don't worry. I'll get to it soon.

I've been using the word 'know' very liberally, and I've placed scarequotes around it occasionally. In the philosophers sense, spiders don't know how to spin a web and much less do their genomes know how to make silk thread. There is no justified true belief here; indeed, there is scarcely anything like a subjective knower in these examples. However, there is an intuitive sense in which we are nonetheless tempted to call it knowledge. It's not abstract or subjective knowledge of a proposition, but rather a tacit knowledge embedded in structure or habit.

While abstract knowledge, like modern physics, has far-reaching (figuratively "reaching" out into the far corners of space) applications, the tacit knowledge embedded in habit, genes, and even tradition tends to be parochial and specific. The significant exception is the genome that "knows" how to build a human brain, because this allows it to exploit the universality of abstract knowledge.

So far I've been talking about adaptations as tacit or parochial knowledge, but I still haven't talked about where they come from. If William Paley's explanation is unsatisfactory, then how can we explain the existence of adaptations? The answer provided by the theory of evolution is mutation and selection.

Mutations just happen. Of course, they have physical causes, but nothing about the causes of a mutation entails that it will be beneficial or lethal to its host organism. There is a sense, then, in which adaptations do "just happen" spontaneously; such knowledge is no less part of nature than tides, fire, and distant nebulae. However, complex adaptations do not happen all at once, but are gradual accumulations over thousands and millions of years.

What needed explaining was not how small random flukes sometimes work out well--we know as much from everyday life. What needed explaining was how something as complex and fine-tuned as the human eye could possibly have arisen in a universe that, prima facie, seems unsuited to its existence. This is exactly what the theory of evolution by natural selection achieves.

The striking thing about this explanation of adaptation is that it bears little resemblance to knowledge in the philosopher's sense, even though the very possibility of abstract and subjective knowledge apparently depends upon such adaptive processes (unjustifiable non-beliefs).

Like I said, mutations just happen. They have physical causes, but nothing about their causes tells you anything about whether they're adaptive, and yet sometimes they are. If this process can, in principle, lead a spider to know how to spin its web, then why might not a similar process lead to useful or even true theories in physics? Maybe something like this process is actually happening through the institutions and methods of science.

Seeking justification, in this view, is like trying to derive the adaptability of a mutation from its causes rather than its consequences (as though one frequency of radiation inherently produces adaptive mutations and another maladaptions). That is, it identifies "knowledge" as subjective beliefs with special causes, whether sense perception, logical proof, divine revelation, or whatever.

The special causes are what constitute justification for the belief, except nobody can agree which justifiers are right--which causes of belief are the "true source" of knowledge. It's from this quagmire that the Münchhausen Trilemma rears its ugly head.

However, for me, this is like asking: which causes of mutation are the true source of adaptations? The question simply doesn't make sense in the context of an evolutionary theory of knowledge, (i.e. evolutionary epistemology in the vein of Popper, Bartley, Campbell, and their students). The Münchhausen Trilemma doesn't make an appearance in this theory of knowledge, because knowledge doesn't have anything to do with justifying beliefs.

All this said, if your friend desires the comforting illusion of certainty--the empowering feeling of knowing, for sure, that he is right and everyone who disagrees is wrong--then I can't help. Maybe he should join a cult or become an Objectivist. I'm fallible and can offer only fallible answers.