You are assuming Adam watched her. The Bible does not say he did.
He was with her.
How could he have not seen and heard what happened?
The Holy Spirit seems to have especially held back a lot here while showing us there was more to what happened, for instance in not revealing more about what it means that Adam was with Eve at the time, so we don’t know what he heard or didn’t, and also that Adam sinned partly in listening to the voice of his wife, though there’s no record of that exchange also.
Interesting too to consider that Adam and Eve might have been able to work together in the Garden until their punishments in being expelled from it drastically changed everything.