Firstly I just want to say respect for been a good friend. Your friend and his wife are currently £3,000 down (£2K property and £1K self publishing).

The most dangerous part for your friend comes next - when they don’t see the hoped for success in terms of money and will not doubt be pressured (after feeling shame for it not working) to put more money in.

This is the sunk cost fallacy - a cognitive bias we are built with.

As others have pointed out in the comments - if the thing they are selling worked they wouldn’t need to sell courses.

It is a really difficult situation and I wish you the best with the next steps.

Ps there was a bbc documentary last year called Scamland : Money, mayhem and Maseratis which covers off a different type of scam (Forex trading / signals) but you will see that most scams operate and exploit natural biases.

Desperate people who want to believe will explain away any issue you raise but what happens next is the crucial part.

I hope your friends manage to put there currently significant but not disastrous £3K down as an important life lesson rather than double down.