Is it safe to trade the world in fortnite?

So is it allowed or is it just scamming prohibited?

Actually, scamming is not prohibited, but can be reported (you should not do it).

Because a friend says that he got a warning because he traded

He was probably reported.

No he was contacted by the support

He's probably been reported, that's why EpicGames looks at this message and then he's probably going to write it down. That's probably how it should be.