Is it possible to really/safely lose 2kg per week?

I got into a discussion with my brother who claims he lost 2kg this week. My argument is that I don’t believe you can safely or realistically lose that weight, unless he has stopped drinking water or he’s running a crazy unsustainable/unhealthy deficit. Note: he is not an MFer.

Upon researching I’ve found non-credible articles dating 2kg a week is equivalent of 500kcal daily deficit but for some reason that does not sound right to me.

Just wondering if anyone else has any thoughts.

Edit: Thanks all for the responses and comments.