I have a question pertaining to US overstays?

If someone was given voluntary departure, no court order, no paperwork just a phone call and warning. travelling through the airports nobody stopped them questioned them or said anything to them. How do they know if they will be penalized or how do they find out if they have a bar against them? Would prefer answers from people who experienced this situation recently. This is my little sisters ordeal. as she was away on scholarship then lost it and wanted to remain in the US she did not know what she was up against and is dying to know if she has lost her travel priviledges to the US.