They can charge her no rent at all if they wish, and that is common in family situations.
However, allowing someone to live rent free or for reduced rent in a property is a gift and my be liable to CAT depending on the relationship. The value of the gift is calculated annually as being the value of the rent that would have been paid for a similar property on the open market. When the value year on year exceeds the threshold amount, Gift tax will become due.
Either way, whether the daughter gifts the E3,000 to the parents or not, if she is paying little or no rent, the rent foregone is still a gift from her parents. Revenue seem to have it sussed.