First, there isn't enough information to know what it is this person has (or has not) done. Secondly, the fifth amendment of the US Constitution allows you to refuse to provide testimony which you feel may incriminate you. Generally encryption pass phrases do not count as testimony, the legal system treats them as keys. And that would be covered under the fourth amendment which says the government cannot compel to yo…
The real problem here is how a judge / prosecutor thinks about digital data as 'property'. If they have possession of the encrypted data, it is up to them how to use it as evidence to build a case or to judge it. You have not to do anything to help them with that as the acused.