Go to hondaautomotiveparts dot com and put in your sons info. It shows a diagram of the crank and attaching parts. The key is $2.50.

Sorry, I can't remember if the pulley or the crank was toothed.

Tighten the crank nut- TIGHT!

That happened on one of my Civics months after I did a timing belt job. The bolt must have loosened up over time.