Hey there @brounoo, thanks for taking the time to get in touch. I'm sorry to hear that you encountered a cheater on PvE while playing The Division 2. To report the cheater going forward, I would recommend you follow through with the steps in this article and use the in-game functions.
Alternatively, you can always reach out through our Support Website directly and create a case. Here, one of our specialists will be able to take a closer look into it. Please do not share any details regarding the player or their username here on the public forums, as this goes against our Forum Rules.
If you have any further questions or you experience any other issues, please do not hesitate to reach back out.