We don’t know if Adam was present when the serpent spoke to her. However, we do know that Adam was present when Eve took the fruit from the Tree of the Knowledge of Good and Evil:
So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate. [Gen. 3:6]
Perhaps we can make some possible suggestions (maybe Adam was curious to see what would happen; maybe he didn’t want to annoy Eve by telling her to stop; etc.), but we aren’t told why Adam didn’t say anything to stop Eve. What is clear, though, is that Adam should have said something to try to stop Eve. That would have been the loving thing to do.
Paul seems to pick up on this in his first letter to Timothy when he says the following:
Let a woman learn quietly with all submissiveness. {12} I do not permit a woman to teach or to exercise authority over a man; rather, she is to remain quiet. {13} For Adam was formed first, then Eve; {14} and Adam was not deceived, but the woman was deceived and became a transgressor. [1 Timothy 2:11-14]
Adam should have said something to help his wife, but he did not.
Further investigation
In the Bible, Adam and Eve are like a picture of Jesus and the Church (Eph. 5:31-32). However, the difference is that, whereas Adam failed his wife (Gen. 3:6), Jesus succeeded in caring for the Church (Eph. 5:25-27): Adam didn’t saying anything to help his wife overcome temptation, but Jesus teaches the Church and leads her away from temptation, if the members of the Church listen to his instructions (Mat. 7:24). God asks Christians to “act out” this relationship between Jesus and the Church (e.g., 1 Cor. 11:3f.; Eph. 5:22-33), with the male Christians representing Jesus and the female Christians representing the Church — hence Paul’s instructions in 1 Tim. 2:11-14: male Christians are to show what Jesus did (which Adam didn’t) — i.e., speak to lead the Church in a godly way — and female Christians show what the Church should do (which Eve didn’t) — i.e., listen to the leader, Jesus. This is like living a parable that is designed to help all Christians remember to listen to and follow Jesus.