The overturn doesn't say she and the other guy did it, it just means the appeals court went too far in its declaration -- just as here in the States, for example, the Appeals court is simply looking at whether or not the original trial was handled properly. In Italy, they are saying the appeals trial was mishandled and a new one needs to be ordered. But I do not think they have yet released the specifics of exactly what they found wrong with the appeals court verdict.
I just find it appalling they can't make up their minds. First they have her in jail four years, then they let her go (so she travels back home to another country) and then 18 months later they're like, "Oh, changed our minds." I mean, seriously? How can they be so wrong TWICE? The police mishandled the initial investigation and interrogated a teen for 14 hours without a lawyer present, among other things; then the appeals court is at the other extreme of criticalness of the investigation; and now the higher appeals court is criticizing the appeals criticisms.
If she doesn't go back, she'll be charged with contempt, which is meaningless unless she hopes to return to Italy at some point. If she is reconvicted, then it will be an issue for the US and ITaly to work out.