Failing to *report* more than $10k cash and filing an official form is illegal. There isn’t really an upper limit, but no doubt the Feds will investigate where the money came from.
For someone even remotely connected to a scam, it might be a Catch-22; illegal to not declare and incriminating if declared.
Evidently they allowed her to make a declaration after they discovered the money. I’ve seen that happen coming into the states and it was a foreigner who claimed to speak no english.