I don't think there is any type of conspiracy regarding the sire to your pup. Your friend most likely, truly thinks the GSD is the father. A bitch can mate to several dogs during her heat cycle. If your friend allowed the bitch to wander the streets while she was in heat, who knows who the sire of your dog truly is.
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA