When we first hear about Olivia running off with her secret lover (Ch.17), the young son tells Dr. Primrose that "she cried very much, and was for coming back;" but the lover, "persuaded her again, and she went into the chaise, and said, ‘O what will my poor papa do when he knows I am undone!'"

That certainly sounds like an acknowledgement of guilt to me! And, she was repeatedly warned about Thornhill's treachery.

The evening before they arrived at their new village, they stayed at an inn 30 miles from their new home (Ch.3). The innkeeper told Dr. Primrose, "that scarcely a farmer’s daughter within ten miles round, but what had found him successful and FAITHLESS (emphasis mine). Though this account gave me some pain, it had a very different effect upon my daughters,..." In Ch.21, Olivia admits that Mr. Burchell, "took EVERY OPPORTUNITY (emphasis mine) of privately admonishing me against the artifices of Mr. Thornhill..."

She then continues in that chapter to acknowledge that Thornhill "owes all his triumph to the desire I HAD of making HIM, and NOT MYSELF HAPPY. I KNEW THAT THE CEREMONY OF OUR MARRIAGE, which was privately performed by a Popish priest, WAS NO WAY BINDING AND THAT I HAD NOTHING TO TRUST TO BUT HIS HONOR (all emphasis mine).”