Fern didn't see Professor Culloden weaving the magic handcuffs, either.

No. All she could see - or think about - was Cuthbert's nasty hand trapping her's. And about how she bet he didn't wash his hands after he handled dung bombs and whatever other questionable objects he kept in that bag of his. And about how dung bombs were made from - wait. Were they made from real dung?! EW EW EWWW!!! The sudden appearance of the handcuffs on their wrists didn't stop her efforts, and Fern dug her heals into the stone floor and pulled, pulled, puuuuuuuulled-

At last! She had freed herself! "WoowoAH!" Fern's jubilant face quickly turned surprised and fearful. She had pulled too hard, and now she was flailing, trying to catch her balance - or catch something to hold onto.

That's how she suddenly found herself clinging to Cuthbert's left arm.

...

...

...

SHE WAS TOUCHING CUTHBERT. HE WASN'T TOUCHING HER. SHE WAS TOUCHING HIM.

Fern let go. She didn't look at Cuthbert. She didn't look at the professor. She wasn't listening to anything anyone was saying. She was planning the fastest route to Azkaban, so she could offer herself up to a Dementor and get her soul sucked.