She brightens because oh good! This is good news even if where they are is currently terrible. She and Magnus had been waxing poetic about their missing loves, and she's so glad that Alec has actually showed.

"Yes!" There's a pause. "Well. Not here in creepy Alice in Wonderland, but in the real world? We're friends."

Is the unwinding the real world? She doesn't know and she's not sure she can really tell him, but she's willing to assume this is a giant mass hallucination.

"He showed me a picture illusion thing." She'll include a nice flourish, just like Magnus usually does with his magic. "Just in case you happened to show up."