The person being searched should have refused to provide any password. US courts have agreed that the right not to be a witness against yourself means that you can refuse to answer questions about your password, since it is forcing you to confess to knowing the correct password to unlock a phone.
I believe that that is what the person's lawyer is arguing, that he was never told of his right to stay silent, so the password is illegally seized evidence, and anything that law enforcement found based on the evidence is also inadmissible in court.