Should a 12 year old have acrylic nails?

My husband earned full custody of his 2 children, now aged 7 and 12. They live with us. Biological sees them a couple hours a week, no overnight visits permitted yet. Yesterday, on a visit, she took our 12 year old daughter and paid to have acrylic nails applied. (she doesn't pay child support at all) I am outraged at the bio mom for not asking my husband or myself, and we think she is much too young for the nails and responsibilities that come with it. I'm totally ok with painting her nails regularly, but I feel like she needs to be AT LEAST 14. Our daughter still goes to a babysitter after school!!! What do you all think?