Here are my 100% Achievement guides for Afterparty!
After helping either FELA or LYNDA, you get an invite and can finally get inside Satans party.
Once there, a bit of dialogue will happen and you see satan the first time.
Once he dissapears, just keep going ALL the way to the right until you see Satans big old dog, simply press
by the dog to pet him, achievement unlocked.
Go to 1.52.05 to see how and where this achievement gets done!
Hope this helps!
Part 1
Part 2