Who told Eve about the fruit?
The simple answer is Adam., why? Eve was created to become his "helper". The "helper" commonly receives instructions and so, since Adam was the first creature who received God's instructions or commandment, Adam has the responsibility to inform his "helper".
"The LORD God also said, “It is not good for the man to be alone. I will make for him a suitable helper.” (Genesis2:18)
It would be wise to think that Adam being first creature who are already doing the Will of God will inform the Woman whom God had given him to become his "suitable helper".
To know more about the Woman's role as a "suitable helper", see this link;
A SUITABLE HELPER (IN HEBREW) Posted by Marg | Mar 8, 2010 | All Posts on Equality, Equality in Marriage, Gender in Genesis 1-3 https://margmowczko.com/a-suitable-helper/
But for us to be convinced that Adam really instructed the Woman about not to eat the fruit from the Tree of Knowledge and Good and Evil is to know what is the role God given to Adam when he placed him in the garden situated in the east.
God had placed Adam in the garden situated in the east, it means that God had placed Adam in the Temple of God, the Sanctuary of God to made and "offering" to worship and glorify God.
CCC358 God created everything for man,222 but man in turn was created to serve and love God and to offer all creation back to him:
Adam role in the garden of eden was a priest, a High Priest to "offer, worship and glorify" God in the wonders of His creation.
See this link;
Adam: High priest of humanity
... Adam and his duty to till (‘abad) and keep (shamar) the garden. These words are better translated, “to serve” and “to guard.” These two Hebrew words are only used together elsewhere in Scripture to describe the duties of the Levites. In Numbers 3:7-8 and Numbers 8:26 the Lord gives the Levites the authority to minister in the tabernacle.
https://www.catholicnewsagency.com/resources/sacraments/holy-orders/adam-high-priest-of-humanity
And this link;
Adam, the First Priest Within the First Temple PUBLISHED ON January 13, 2016 by Adam Powers https://thepublicans.org/2016/01/13/adam-the-first-priest-within-the-first-temple/