My personal opinion based on years of dog adoption is that all members of the family who will give commands should take part in the training. Dogs are way smart and will use that as an excuse to not do some task if allowed. Both of you need to commit 110% to the outcome to get a good result. Training a GSD is definitely not for sissies, they will exploit any flaw in your program and turn it right back on you, so be firm, fair and above all consistent both singly and jointly. Both of my girls are scary smart and exploit some flaw I commit every day, when I catch it and close the loop they just look around and find something else. It has become almost a game, but I love my girls an awfull lot and they also know once I catch on the game is up...So if you and the BF are getting a dog he had better join in or be relegated to the level of "Toy" in the pack and never be taken seriously by the dog,...Ever.. Good luck!!
Hally, The Olden Golden 12/5/1999-
Indi, (GSD) 8/4/2011-
Dixie, (GSD) 8/11/2011-
Sassy, (Rottie) Born 10/9/1999-6/12/2006 RIP