I just purchased the BG2 Complete edition from GoG, and I was wondering if it's possible for a friend to download the game as well and play without DRM issues. If this is against any rules or any such thing please let me know as well as I would like to respect GoG as well as the owners/creators of BG2.

I think that what you are asking is answered in the 9th question of Website Help>Downloads and Games FAQ.

Basically you can install it on any household computers, but not share the files with friends. That being said GOG does not have any DRM installed in their games, so they basically trust you that you won't share the files and don't try to enforce it in any other way.
But then again if you wanted to pirate a game that's being sold here, you could easily torrent it and not bother with GOG at all.