Since discovering what exactly she was, Lydia had plunged head first into trying to learn her complete abilities. She was finally starting to hone in on things a little less sporadically when she was shocked to her core.
Someone was trying to contact her.
The voice was initially muted because whoever was attempting to contact her was already dead.
The two never had real close interactions when Erica was alive, but they were both connected to each other in many ways. That was how they were able to find each other with Lydia's gift.
The communication was still one way cause Lydia hadn't figured out how to communicate back, but the running commentary she was getting was keeping her in stitches.
On the drive the Mexico, the one way conversation kept her grounded, while Erica slipped tidbits to help intimidate the opposing forces that she would be facing.
Erica was the one that kept her from freaking completely out when she was attached to the electric torture chair. She was the one that told Lydia that she needed to believe in her Alpha, to believe in Scott.
The snips that Erica made about the "new girls" in the pack were silly, but it was when they were confronted with the de-aged Derek that Lydia couldn't control her response.
Lydia chuckled to herself. It was probably the most inappropriate time to chuckle, but she couldn't help it.
"Damn Lyds, baby Derek is a hottie."