First she must make sure that she has in fact become 'illegal'. I know that there are some areas of a student visa's eligibility that may not be so clear cut.
If she is in fact out of status, and she enters the lottery and wins then she will not be able to adjust her status in the US and if she leaves the US to do CP then depending on the length of her overstay, she will be banned from re-entering for a number of years.
If she is in status then she should make sure that she will be in status until september 2012, in order to claim a visa if she wins.