You've GOT to get a good mark on this. The last couple of tests really haven't gone your way and you need to bring your mark up. I'm not sure that I like the idea of buying someone else's work and passing it off as my own, but if I've bought it, it is mine, right? You get on the internet and start searching.
Within a few minutes, you've found one of these services and they seem to have essays on your subject. You pick the one that seems to be the best purchase and click on the "buy" button. You skim it, it seems good. You put your name on it, print it and it's ready to hand in tomorrow. Time to get a good night's sleep.
This can go one of two ways: they get away with it or they don't. There's likely a good chance that the teacher will notice the inconsistencies between the student's usual writing and this paper. A quick Google search of a passage may well turn up the original essay, this is assuming that the school isn't using a more sophisticated plaigarism checker.
The best long-term scenario is that they get caught. This gets ugly and painful in the short term, but they at least potentially get to mend the error of their ways. If they don't get caught, they have learned that this may actually be a viable option in the future and repeat the offence. Either way, it is unlikely that they will have met the intended learning outcomes of the assignment.