She should check with her school to make sure there aren't any requirements regarding having taken or passed the CS before graduation, as well as checking with her residency to make sure they don't require that the CS has been taken or passed before starting the program. At this point, if she takes the CS before May 17, she wouldn't get her results until June 18. If after May 17, then the results wouldn't come in until August.
Available spots for the CS are pretty much filled through the next few months, but there are random spots available if you aren't picky about where you take the exam, or taking the exam during the evening session.
At this point, if she's slated to graduate this spring, she's all set on the clinical coursework she'll need for the exam, and wouldn't need much more preperation than a weekend with First Aid for the CS. I would think that she should take the CS as soon as is convenient for her.