My compliments to the person who told the manager to stop using the term. I support that.
Even if there were no historical basis for viewing the term 'ladies' as a derogatory one, the fact that it offends one employee enough for her to talk to her manager about it should be enough for the manager to stop.
I would coach the manager that a better way to handle it would be to say, "I understand it offends you when I use the term 'ladies,' so I will stop. I really appreciate that you came to me and let me know."
Then the manager wouldn't talk to any other employee about the request or get other people's input. She'd just stop saying 'ladies' when she talked. Remember: the term 'women' or 'professional women' is always safe in the work setting.
You notice I didn't say, "Try to stop." I said, "Stop." My rationale is that stopping use of that word would have no negative impact on the manager's ability to do her job or motivate her employees or anything else whereas continuing use of the term would show that she doesn't care enough about an employee's comfort in the workplace to accomodate a simple request.
This employee isn't alone in her negative reaction. I have heard many times over the past 20 years that the term 'ladies' offends many women because of the historically used term 'ladies of the evening' for prostitutes. In fact, Jane Elliott, who is a well-known speaker on diversity issues and the creator of the well known video, The Eye of the Storm, mentions it in every speech I've seen her do. Her advice is that if you respect women, quit using the term.
A long answer to a great question. Any more questions, feel free to write.