Per the Supreme Court, no one is required to carry ID in the US on a general basis. Police cannot randomly come up to you and ask to see even your ID, let alone your passport. I have never heard of security checks on Amtrak trains in California, but even if there are, your friend is not required to show them any papers or answer any questions. There is a small chance that they might do a check for drugs or guns, but not for immigration. Even at the San Clemente checkpoint, which is close to the Mexican border, US Customs and Border Protection cannot check IDs of anyone except the person driving.