The disorientation will help convince her that it was all a dream.
3 seconds sound clip from the Back to the Future Part II movie soundboard.
You can hear this line at 00:39:27.273 in the Blu-ray version of the movie.
Quote context
[...]
- Did we make it? Are we back?
- Wait a minute. We're just gonna leave her here on the porch?
- The disorientation will help convince her that it was all a dream.
- How long do you think she's gonna be out?
- You better bring smelling salts back with you.
- All right. Come on. Let's go, Einie.
- Don't worry. She'll be fine.
[...]