It's not your choice
If they're going to pass out, you can't prevent it.
What you want to do though, is if you think they're going to pass out make sure they don't fall and hurt themselves. Help them gently to the ground. Our 1st instinct when someone is going down is to try to help them back up or to sit them up. Don't do that. The reason we pass out is so our head goes down to get more blood to our brain.