The fact that he was validating her, despite being abso-fucking-lutely horrifying in it's own right, at the very least, Lisa's panic was starting to subside. That she knew what was happening to her wasn't just in her own head was a very effective drag back down to rationality. With that, came a very tense sort of calm.
Of course, the gravity of what Sam was telling her had it's own horrible weight.
"Wiped," she parroted back, skeptical only because how the hell does that happen? But, here they were. She knew he was telling the truth, because there was literally no other explanation for the fact that she loved Dean, yet barely had a two second memory of him from her life before Marrowood. "How... why?"