Fine... she didn't need to tell her Mom - so what? Does that mean the mom should have said nothing? At 18 she may be legally old enough but that doesn't mean that the mom should idly and quietly sit by when her daughter is about to do something totally stupid. I would hope that given what ultimately happened, the mom is able to say that she never knew what was going on because the daughter hid it from her until she was well and truly sick .... or she knew but couldn't persuade her daughter otherwise. Otherwise, she knew and approved of the initial piercings - and for both case was clueless on reading the signs when the daughter got sick. Both of these are not the signs of a decent mom.
KIds do what they want anyway...and ask forgiveness after the fact. You're lucky if you get any sayso over what they do. At 18, she might not even have lived at home. You certainly don't have any legal control over them. Frequently your only choice is kick them out or deal with it when things hit the fan.