Yahoo messenger invisibility?

My girlfriend is having a problem. Whenever she logs onto yahoo messenger, an ex-boyfriend is able to tell that she is online even if she is invisble. I’ve checked her profle and she isn’t shown as visibile, in fact everywhere I check she is listed as offline.

Is there a flaw he might be exploiting oris there something that he may be running in order to see that she is online? Any ideas/input would be appriciated. She is running win 98 second edition. Thanks, William