CT aside, I personally do not think collars should be used with leashes.
I agree with the above... please do your best to switch him to a harness. I have seen too many dogs, family and friends, that have slipped out of their collar and been run over, run off from fear of a loud noise or attacked by larger dogs.
But, congrats on outside potty!! That is definitely worthy of celebration!
Also, I'm glad that the trainer told that GSD owner about the shorter leash. I hope it sticks with her... So many people are just ignorant (and/or arrogant!) of that fact. Yes, you know your dog...but you don't know your dog's "thought process"/instinctive drive. You don't know what will happen or how rough they'll "play."