From my experience, you do need a return ticket. I don't think as an American citizen they will let you board the plane in the states without a return ticket. How I did it was get a ticket that was open-ended, it was a return ticket but was not dated. So, I'm not sure about the 3 month deal unless she is making reference to as a tourist, you cannot stay longer than 3 months without an extension. That is a big deal, as a tourist you must leave the country after three months if you do not get an extension. When I lived there some girl just ignored it and stayed longer than three months, they were not happy with her. Not sure what the outcome was. One time I was one day late getting an extension and they could have given me a hard time. If you are in the country longer than three months, then want to leave, forget about getting through customs, you cannot do so without that extension.
So, it is something to be taken seriously.