who can save Lilly?

When I ask this question I can see that practically everyone can save Lilly at various points in her downfall. In particular, I think that the role played by Seldon is despicable. He is her closest friend and yet it is he who sets in train a series of events which lead to the downfall - commencing with their first meeting in his rooms and his contrivance to keep her from meeting Percy Gryce at church. At the end of the day he does not own up to the part he played in the downfall and nor does he do anything constructive to help her. As for Rosedale, he understands entirely that Lilly is not to blame for any of the charges against her and yet with that knowledge he also fails to do anything about it. She is portrayed as a flippant, flighty party girl but by the end of the book we see that she is the only real good person (perhaps bar Gertie) and the others are ugly parasites who drive her to her ultimate fate.

That's a fair comment, but Wharton stresses that Lily has freedom of choice and is always torn between two movements, centrifugal and centripetal.

Unlike Selden, she'd really like to be part of the showy high society, yet with him, feels contempt for their shallow values. No one is to blame for her decline. She is blessed with beauty and intelligence and opportunities fall at her feet that we'd all pray for. Lily's tragedy - and its the genius of Wharton that she shows us this in the novel - is that, like MacBeth, the crown she wants 'highly' she would have 'holily' and 'would not play false'.

Unfortunately, in the real world, you have to get your hands dirty and Lily can't do this. Selden isn't to blame: he represents a social position that, like Gerty Farish, allows him to criticise the carnival. This is a judgement play, like Merchant of Venice. No one - repeat, no one comes out of the story well.