Having a friend mediate a resolution is fine, if it works. Litigation is never 100% predictable, and it’s not something to do, if you don’t have to. Also, we don’t know or need to know what “done wrong to him” means (if you know). Your post appears to suggest that the person to whom the loan was made is now looking for reasons – perhaps all specious – to not pay as promised.
If that’s the case, and the other friend’s intervention doesn’t work, you go to court. If that happens, the emails are better than a handshake and a promise and a loan made in cash. They aren’t as good as a written agreement that you both signed and a record of funds transfer. That was the point.
If you go to court, you have the burden of proof. You must satisfy the court to at least a 51% level of certainty that a loan was made and that, by its terms, it is now in default. If the beneficiary chooses to say that it never happened, or that it was a gift, or that it was already re-paid, rebutting that falls to you. If you can’t meet your burden of proof in any event, the beneficiary doesn’t have to say a thing and you still lose. Therefore, you make your proof as strong as possible, even if it’s a tightly wrapped spider’s web, because that’s all that you have to serve your needs. Fortunately, SC is generally a bit more lenient about what it will consider as evidence.
And, if a friend can solve the problem and obviate the need, all the better.