Nobody is suggesting that this is something you can't say, to anyone, in an appropriate circumstance. The author is merely explaining why it was difficult in this case, and how it made her feel.

If the context hadn't been in front of a client, perhaps she would have been able to ask for clarification of what was meant - and the whole thing wouldn't have cause such a big, hidden reaction in her. The context made this more difficult that it should have been - it is not saying "don't ever say this or you're sexist", that is what you've read into it yourself.