Conviction of a crime with a maximum possible jail time of more than a year can lead to deportation. If the person was convicted of a felony, chances are they would be put on deportation when they try to renew their green card or apply for citizenship. It is not your job to inform them..............But they don't mind getting help. If you can live with the fact that you triggered the person's search and deportation, then the call is yours. If immigration finds the person, he/she would be arrested and trigger deportation hearings..............