Political campaigns are all about exposure, and people who run for public office must expect to see their past deeds and misdeeds paraded in the media. Spin sometimes takes bits and pieces of shady activities, propelling them into the public arena with appearances much more heinous than they may actually have been. Candidates leave it to the masses to sift the dirt to separate lies, truth and gossip. All is fair in love and war, so they say. No one would equate politics with love, so as the candidates brace for war, we expect Clinton to use any ammunition she can get to shoot down her opponent.
Reading about Trump's lifelong objectification of women, I am doubly looking forward to seeing Hillary Clinton cut him down to size. She, as a capable, accomplished woman, will not retreat from his sexism. That would be poetic justice for The Donald's lifelong arrogance around women. Making unwelcome advances toward women is a sign of an insecure man always trying to prove to himself and to others that he is worth something.
No, Clinton is not crossing the line asking former Trump writer for dirt. Trump has done the exact same thing to discover things to use against Clinton, attention just isn't drawn to it. Clinton and Trump should be held to the same standards in their campaigns. Trump is doing much worse things.
If you decide to run for office, information about your life and you business dealings are fair game, as long as they don't delve into things like candidates' children. The person who wrote Trump's book may well have information that the American public needs in order to make and informed decision.