Matt listened. He noted her already soft and slow heartbeat flutter as she spoke, the way her words caught in her throat... This friend, obviously, was her. But he didn't move to interrupt her until she had explained as much as she was willing. And he knew she wasn't lying per say, but that there might be more to the story that she was withholding from him. He had been there before, and he had experienced people only giving him a half truth.
"Liv, I have a feeling that there are vampires in this City. Are you trying to tell me that your friend is a vampire and that she might not be going through the proper channels in which to satisfy whatever needs she may have?" As long as nobody was dying, Matt didn't care what she was. And as long as she steered that hunger clear from Rose then they wouldn't have problems.
There was still no judgement in his tone, only a mild curiosity coupled with an honest statement. "From a legal stand point, that wouldn't fly in court. Ethically, I have a feeling that as long as your friend isn't murdering innocent people then she shouldn't feel so worried about exposure."