You're making the assumption that he was there by her side approving of what she did. "With her" is not constrained to such rigid meaning.
No assumption, merely reading what it obviously says and means,
although actually the first or original sin was not eating physical fruit, but rather choosing not to doubt the Devil,
not to cooperate with God, and not asking God why the serpent contradicted what He had told them (Gen. 3:4, MT 7:7).
It is wise to question God. The serpent introduced the option of ungodliness or functional atheism, which they chose.
in Gen. 3:7-13. Adam and Eve tried to cover up their crime, refusing to accept responsibility for their mistakes. They should have confessed immediately (1JN 1:9). Instead their ungodly attitude was manifested by a chain of sins, including their silly attempt to hide from God and to blame someone else and very likely “dysfunctional” parenting that probably contributed to the emotional disturbance (anger) in Cain that resulted in the murder of Abel and so on and so forth through the millenniums.