If i noticed a nurse sleeping on the job and mentioned it to another?

co worker and weeks down the line she goes and reports her sleeping to the supervisor and she says that i saw her in one of my patients rooms later im being told to write a statement of what i saw or i will get written up. wouldn't the other girl be at fault for reported her that fact that she quite her job

If I'm understanding what you said correctly (the way you worded is a little challenging to follow), it looks like your supervisor is trying to make sense of what, exactly, happened before deciding what to do about it. You aren't being accused of doing anything wrong, but you were cited as a witness, and, therefore, a source of information. The very fact you're being asked to make a statement means that your supervisor is giving you the benefit of credibility.

Write down anything you may have seen. Be honest, be thorough, and be as detailed as your memory allows. Include a statement about why you did not bring this to your supervisor yourself, as it'll be easier to write that down than to have to explain it verbally on the spot.

One way or another, your supervisor will have to come to a decision, and it sounds like s/he is trying to be thorough. The more helpful and forthcoming you are, the better of a light you'll be judged in no matter what actually happened.