What to do when a friend loses his crown?

Don't use superglue! If you mean "lose" as in cannot find, then you'll have to see a dentist to make a new one. If you mean "lose", like came off the tooth while eating and I still have the crown in my possession, then don't use superglue to replace it. Drugstores usually have temporary glue you can use, but eventually you'll have to see a dentist to get it replaced properly. This is much less costly than a new one.
See a dentist. Take your friend to a dentist, he will need to have the crown replaced. It's a very simple procedure. If you have the old crown that fell out you should bring that with you- don't throw it away!