For starters I would remove and have the holes heal up seeing as the plan was they weren't to be done until she started high school
I'd also be having a serious talk with dd about NEVER keeping secrets from parents. She is far too young to be not telling her parents anything as important as this.
The gf - I hardly know where to Fucking start!
Firstly I'd be putting her on notice that she is NEVER to do anything that constitutes a parental decision without at the very least discussing with dds dad in the first instance.
I'd then be asking her f2f WHY she never told ex OR you given that such a decision required follow up care for the child's MEDICAL safety. An allergic reaction or infection for example wouldn't necessarily have been centred on the ears, it's entirely possible dd could have developed an all body rash, anaphylaxis or very high fever or worse WITHOUT you having an inkling of why which could have affected medical treatment to dds detriment- I would absolutely HAMMER this home. Ask her "how would you have felt if dd had become SERIOUSLY ill as a result of this and we the parents didn't know to alert medics to the possibility it was down to a piercing?! Do you even know dds full medical history so if any reaction had been instant and occurred in the shop you'd have been able to inform medics of this including all meds she is on and past allergies?"
I would (given this isn't the first time she's pulled shit like this!) follow up with solicitors letters to both the girlfriend AND the ex being very clear that major parental decisions are NOT to be made without consulting you first.
I'd also include in that letter that encouraging a NINE year old to keep important secrets from parents is a safeguarding issue and potentially very dangerous and is to stop immediately - no ifs buts or maybes
She has MASSIVELY overstepped, potentially dangerously so in a number of ways. I would be absolutely livid!
I think this is one occasion you absolutely can (in a controlled but firm way) go "mama bear" on the girlfriend.
She has behaved irresponsibly and arrogantly.
I agree too that there is potential - if not nipped in the bud - that the girlfriend is potentially the type as your dd gets older for even more inappropriate behaviour like supplying alcohol/drugs, encouraging under age sex etc as dd gets older so it needs to be firmly made clear to the girlfriend that these are NOT her decisions to make.
I am very fortunate in that my dds step mum was a former friend and babysitter who knew dds medical history. This came to be very useful on at least one occasion when my ex (who is a useless fucking nightmare!) failed to mention dds asthma when she took very unwell with a flu while under their care.