Even though the means was immoral, the poor child was a victim. You should treat the child well. If her own ovum was used, it would be fine to refer to her as the child's mother and the child as her daughter. Yet you can never treat them as if the two Lesbians are married or that the other one is a "mother" by marriage.
My wife had a roommate in college who came out later as a Lesbian. At the time, she told her, "Well, I have to tell you what I think about your lifestyle ... but I still care about you." She said it in a compassionate way, and the woman completely understood and did not get irate or combative. Perhaps that little seed is there and more effectively planted than if my wife had stood up on a chair, pointed her finger, and exclaimed, "God hates fags." (like that one Prot group). While one should not actively condone the immorality, St. Thomas also lays out rules for when one should (or even should not) "rebuke the sinner". It depends in large part whether you perceive that the rebuke would enlighten them or harden them more in their sin. It's a question or prudence. I would not visit their home and be "received" by them as if they were some kind of couple, but then I would have the same approach to a couple of cohabitating heterosexual fornicators.