My iMac 8.1 is partitioned into separate accounts for wife and kids. If I power up / use it / put it into Sleep mode, that works. If my wife's account has been used and she has logged out and I try this sequence in my account, then the computer goes into Sleep mode for a few seconds before popping out again. Repeatedly hitting Sleep doesn't help. It sounds like a software issue on her partition carrying over after she has logged off. Why?
Try going ti system preferences>accounts, open your account>login in items, get rid of them. Also try going to energy saver and make sure wake for network access is not clicked. Do this on the other account as well. Hope this helps.