I know there have been a lot of posts about this on the forum and I went through them but none of them have a solution to my problem. The friends sidebar is missing for, and so is the arrow button that expands it. I have it enabled in settings and GoG Galaxy is over 1900 pixels wide for me (so it passes the 1500px requirement).

See attached screenshot.

Thanks guys.

Image: i.imgur.com/vRr8Dno.png

Edit: Sorry didnt mark it as a question, new here.

That seems to be definitely a bug. In my GOG Galaxy that arrow to expand/collapse the friends sidebar is available when I activate the option. I recommend opening a support ticket for this.

I opened a ticket but the issue fixed itself in the mean time, this is what caused it to get fixed if anyone else encounters it:
I turned on Counter-Strike: Global Offensive (a Steam game) through GoG Galaxy and when I closed it the right menu was there. Not sure what the cause was but seems fixed now.