Do you get banned in Fortnite?

Are you banned if you insult an opponent very hard in creative mode and they report you?

Ps: question for my friend

No.

It depends on how severe the insult was and how often it has happened. There may be a temporary ban, for a few days. If it has happened too often, there may even be a permabann.

If it were the first time?

Is still dependent on the severity of the insult or the verifiability, but probably not.