I am not licensed or currently admitted pro hac to practice in Florida, so I reviewed the most recent edition of the Rules Regulating The Florida Bar ("RRTFB") dated March 29, 2019. Subject to correction by someone who is so licensed, I believe Chapter 4, the Rules of Professional Conduct, permit a Florida lawyer to charge reasonable fees only to actual clients; i.e. persons with whom an attorney-client relationship has been created and to whom the amount and basis for a fee has been communicated in writing before or within a reasonable time after representation has commenced.

You are correct that we do not know more than the poster stated: "She called a Florida law firm for advice. They told her they couldn't help, but sent her a $400 bill for the phone conversation." Perhaps there was an existing client relationship with the firm for other purposes, with an agreed hourly rate, in which case $400 for a somewhat lengthy telephone conversation could have been reasonable. However, the implication is that this was a cold call, not resulting in an attorney-client relationship, for which a bill was nevertheless rendered. If so, irrespective of whether the caller was a Florida resident, in my opinion (and I think also of the Florida bar) that was improper and worthy of criticism.