If you were more closely tied to this family, I'd say it was something you could bring up delicately. But as it is, I'd say it's probably not your place.
This child may grow up really upset that her name was changed/taken, or not care in the least. It's hard to know. It's also hard to know the motivations behind the name change, or if the adoptive parents will still make use of the name in some meaningful way (if not in her legal name, then in conversation, memories, lifebooks, etc.) during the child's life.
I think there are some adoptive parents who really think they can wipe the slate clean, and pretend that the children they adopt didn't have a life before the adoption.
Re-naming can be part of that illusion. Other people, like one of the OPs, have well-intentioned reasons behind the name change...those people are doing it, for better or for worse, for child-centered reasons.
Personally, I believe that preserving the name of the child, no matter who gave it to them, is really important. It's a huge part of their original culture and life, and a part of their identity that we can respect and incorporate into our lives (rather than erasing it with a name change). It's a very obvious way of saying/showing that we know they had a life before us, and that life is important to us. That being said, we gave our daughter an additional first name and our family name, keeping her entire Korean first/last name as her middle names. We wanted to give her our family name for obvious reasons, but we also wanted to give her a first name that had personal and family meaning to us. The way her name is arranged:
Englishname Koreanname KoreanFamilyname EnglishFamilyname
...will hopefully allow her to legally use whatever combination of names she prefers in different parts of her life.
Everyone seems to do it differently. Sometimes the motivations are icky, sometimes not...it's hard to tell without knowing the adoptive couple well.