Your friend has found a hole in the system and is exploiting it to make a financial profit, and that is neither legal nor moral. What you're actually asking is whether someone can find people who do it or it can be done without sanctions - and it all depends on how good your friend is at covering his tracks. If someone is caught stealing, it doesn't matter how they did it - the punishment depends on the gravity of the crime and the law in force in a country.
He found several sites, that can be used to make money.
For all those who might want to take advantage of the services mentioned by OP, keep in mind that the whole story may not be true - do not use unverified sites because instead of double BTC you may be left without a deposit. Whoever made a mixer without protecting themselves from double-spend is far from a professional who knows what he is doing.