1. If there is a condition in the probation prohibiting your friend travel, typically may be worded as "not leave state" then he or she is better off not travelling. If he or she is still not a USC then it may be prompt condition for deportation (or removal, as CIS likes to call it).
2. If there is no condition in the probation (or bond) then he or she should get a letter (I am forgetting the technical term) from the court about the disposition of the case during travel. The reason being that the courts DO report non-USC dispositions to CIS. This information is accessible to the INS folks. In absence of the court certified disposition, your friend may be subjected to undue harrassement.
It may be a different issue that CIS is too overloaded to start deportation proceedings against each offender reported by the courts.
Hope this helps