A lady at work said she's on a yo-yo diet. Isn't that bad for her teeth? And how does she digest the string? Yuck!

i dont think thats what they nneant

It's probably a wooden yo-yo.   They have lots of fiber.  (FYI - The string is not meant to be eaten.  It's to be used as dental floss AFTER the meal.)

Sweet and sour yo-yo (sweet when its up, but sour as it goes down)
Yo-yo with Béarnaise sauce (just don't tell Bernie you used her sauce)
Yo-yo al-la-mode (once a sleeper dish, but now it's gone "round-the-world")

It's bad for her teeth, but even worse for her liver. The string would probably be best served chopped...in tiny pieces. 

Swallowing the yo-yo part would stop her from swallowing much of anything else.  It could work if the yo-yo was plastic.  I'm sure she will feel very full because trying to digest plastic would take a while.
I certainly would not want to be on the other side of this, either.  
That is going to make it very hard to pass anything else afterward.  Unused food will probably just fall out.  

It certainly is not a healthy diet and I would not suggest it to her or anyone else.  The health hazards are way to high to weigh-in as a good diet for anyone.