Cisco's more than a little glad Sirius knows Barry, because people like Barry, so being able to tell Sirius he's Barry's friend is an alignment that will likely only help him.

"That's him, yeah," Cisco says with a fond smile. "Barry's my best friend," he tells Sirius. "Like, from home. He was my best friend before we ended up here." He doesn't know how forthcoming Barry's been about his powers here, so he doesn't mention how they knew each other before, just that they did.