I kinda bought the 'she lost her husband and was vulnerable and then got taken advantage of' because, as I recall, the game seems to stress how much she loved her husband and how lost she felt without him. Given that, it seemed to be that Kuja was able to take advantage of her and appeal to her greed to conquer neighbouring countries. I could imagine that, given her vulnerable state, she wouldn't have taken too much convincing and the fact that someone as manipulative as Kuja took an advantage of her and likely flattered her, made her particularly susceptible.