The DoJ doesn't typically claim as far as I know that they will stop prosecution in other countries.
I wouldn't call ~20 years into the _future_ "already done". But definitely think long and hard about any agreements you sign in an adversarial situation.
TBH, the guys not even an American so I don't think he really has a right to stay in the country after his sentence is over.
[1]: https://en.wikipedia.org/wiki/Paul_Le_Roux#Trial_of_Le_Roux
Allow Caroline to testify in exchange for lenience. Share that... so that at some point in the future she can be extradited. All good now! As far as I can tell your criticism has no practical bearing on achieving these ends.
Or you might want to be confessing to a country you're a resident of so that you have actual standing to oppose being extradited.
Maybe there is some way she can avoid extradition from the US, I'm not sure, but I have a feeling such a greedy soul could be lured to some other country in promise of riches only to be escorted by men with badges for a nice cruise for a Caribbean vacation.