I work in a complaints department so deal with numerous emails/letters from customers a day, some of which mention staff.
We have a very strict policy that we do not accept abuse in any form - so any comments along the lines of 'that stupid cow' 'that useless man' etc - get a very firm response along the lines of 'Our staff are highly qualified and always try to provide excellent customer service. I am therefore disappointed that you chose to refer to the staff member in question in derogatory terms and must inform you that this will not be tolerated, should there be a recurrence'.
However - that's actual abuse. The 'descriptive' terms we get vary - 'the blonde girl', 'the young lass' (who happened to be the branch manager!), 'the bird' , 'the skinny guy', 'that lanky man'.
None of which are professional, none of which are PC or really 'appropriate' to use. BUT...customers have the right to use whatever 'descriptive' terms they like, unless outrightly abusive.
You're not on a one-woman crusade here to correct language that YOU find inappropriate. By all means, correct a colleague or friend on the inappropriateness of 'bird' but it's not your place to attempt to educate your customers - the people who pay your wages.
For context, I'm a manager within complaints and if one of my team responded to a customer letter seeking to correct them in anyway, based on their own personal feelings, I'd have them in a disciplinary hearing before their feet touched the ground.