But of course seducing a girl would ruin her life, unless she was brought up to professional courtesanship... or was a member of some profession like an actress who mgiht be expected to take lovers.. Willougby knew that... all men knew it.... That is why it was wrong of him to seduce her and VERY FOOLISH and wrong of her to give way....

As for her situation, I would imagine that Willougbhy knew enough about Eliza to know that she was at least a "borderline" lady, like harrite Smith. So although she wasn't quite his social equal she was still "ladylike" enough that he should have considered her as "not fair game" even if she gave off signals (lack of chaperonage, going about with a girl who probalby covered for her etc)that she was not fully "ladylike" and respectable....
W should have respected the fact that she was ladylike, and that she was very young, that she seems ot have lacked proper protection from her elders, and that (I assume) she was also of a passionate nature which was easily led astray....