Caroline realized that all this time the reason they couldn't find Lizzie was because they were trying to search for a siphoner. Lizzie was a heretic now and she had magic. She could literally siphon her own immortal body like a battery to do spells so it all made sense. Caroline knew that in the end, even if Ric didn't want it, that they couldn't change Lizzie back, they wouldn't be able to unless they had the cure and they didn't.
"We have to tell your father."
She had to. She knew that Lizzie didn't want to face him but they couldn't keep this from him and right now she knew that Ric was out there making his rounds, searching the apartment and the city for his daughter who was still missing in his eyes. It didn't take her long to send Ric a text. Caroline knew that he would be home within minutes.
"When he gets here...you have to explain to him what happened. You need to tell him exactly what you told me." Because Caroline knew that the moment Ric found out about Lizzie's vampire nature everything would change. The former hunter wouldn't be a former anymore. He was going to kill vampires just to feel better about what happened to his daughter.