UPDATE: The firing was still done as nicely as possible, and I still have both mother and daughter as friends.
Without dwelling on the negative aspects of Girlchild's performances, I wrote two e-mails, one for daughter and one for mother. In daughter's, I expressed a concern for her schoolwork, and the fact that she seemed to be spreading herself too thin. I gave her an invitation to come and sing with us as a "special guest star" at certain gigs.
In mother's I explained to her why I did as I did. I didn't want to lie about what the band is doing, I simply wanted all to be happy with the inevitable fact that daughter cannot work with us full time any more.
Ugly things were said.
Tears were shed.
But, logic won the day, and I think we're all happpy with the outcome.
Thanks, 'Catters, for all the good advice and concern.