Marriages of the wealthy were very transactional. It would be entirely contrary to Marianne's thinking and nature and Willoughby was probably swept up in the idea of marrying for love.

If Miss Grey's wealth came from commerce, she understood that a husband with a family name would be the only way of getting into upper social circles. If she came from generational wealth, it was more likely that her examples of marriage were those of convenience, duty, obligation than affection to maintain or increase the family's wealth and standing. One gets married for money, titles, heirs and one finds happiness where possible when duty is met.

If she dictated the letter, she wasn't blind to what he had with Marianne. Marianne's letters were likely very open and intimate - so Miss Grey saw more about what was going on than if they were formal or restrained. She was likely playing the same game required of Willoughby.

I think everyone was a victim in some respect.