Check 1: Did you enable the option to allow downloading videos for this particular friend? Settings > Users & Sharing → Restrictions
Check 2: Has your friend an active Plex Pass membership?
From what I remember, Plex have announced last year that the download feature will in the future only be available to Plex Pass members (while in the past this only required a Plex Pass for the server owner). https://support.plex.tv/articles/downloads-overview/
All is good except friend dont have pass but the weird thing is when my server is install on my pc that work very well to download (and i dont have pass at this time)
I have the same issue.
I have Plex Pass on my server and added my friend as a managed user, and we figured out that he is not able to download items in his desktop app or even in a browser. However, another friend, who was added to my server a few years ago, still can use Download feature. What is wrong here?