The way I understand it is like this:
IF you are born with dual citizenship, (Say, a US serviceman father serving in Korea, married to a Korean National - the child being of an American citizen serving overseas.) you may retain dual citizenship.
However, if you are born in the USA (natural-born citizen) and swear allegiance to another Nation (by taking their citizenship) it is a de facto renouncement of your US citizenship.
I could be wrong here; am researching.
I doubt she’s lost US Citizenship.
Big deal I say. She wanted to play but wasn’t good enough for the US team so she went to a team that would take her. She’s not shelling Georgian villages.