Whoever told her that is absolutely incorrect. She should make an appointment to speak with her priest or deacon or someone who handles annulment at the archdiocese. There are also usually seminars set up once or twice a year in most areas. My recommendation would be to find a procurator to work directly with her. This may be a deacon in the parish or a lay person.
There is no guarantee that an annulment is granted, but if the only reason she has not pursued it was because of the existence of children, then she was misinformed and should speak with someone directly involved in the process.
An internet search can give lots of information about potential reasons why a marriage might be null, but the best advice will come from whomever she chooses as a guide. Annulment is not quick, and it is not easy, but it is worth the effort.
Love,
O2