The Jews warned Pilate that if he released Jesus, he would be no friend of whom?

Answer: Caesar

They shouted that anyone declaring himself king defies Caesar, so releasing Jesus would make Pilate no friend of Caesar.

“From then on, Pilate tried to release Him, but the Jews kept shouting, “If you release this man, you are no friend of Caesar. Anyone who declares himself a king is defying Caesar.””— John 19:12 (BSB)