She threw him under the bus because the police were already on the trail. Instead of arresting both men, the police stopped at the nephew and let the husband go.
It is implied is that her childhood friend, the policeman wanted to give the husband a second chance and stopped the investigation at the nephew.
Guess it's been a bit long for me to remember the nuances.
Whilst I agree the police were on the trail, and were suspicious, could they do much without evidence?
I'm not sure they were planning to act on anything.
It is difficult to see where this will go.
As in clearly the pair will try to board a ship for America together with the evidence.
But it is difficult to see whether her actions would benefit or jeopardise their escape and cause more increased scrutiny when they try to leave.