Depending on the friend, I'd respond one of two ways:
There are a few female friends of mine whose opinions I trust so much that I'd want to know why they feel that way, and I'd seriously consider their opinions.
For any other female who made that comment to me, my response would be something along the lines of "Are you jealous?"
I'd go with that, but don't say "Are you jealous?". I had a school friend who though her younger sisters fiance was not good for her, and said so, and the family accused her of being jealous and not wanting to see her sister happy. They did get married, and he was unfaithful to her, and tried to blame her (If you hadn't taken the kids to visit my Mom, I'd never have done it)