My friend came home to find some papers taped to her front door. I was under the impression that whomever was serving the documents had to attempt at least 3 times before they could post it on the door or mail it Registered Mail. I am not sure how this works. She lives in Pinal County, AZ. Please shed some light on this for us.

What was the notice about? Since you posted under the Real Estate Law forum, I'll assume it was a vacate notice.

This is from: http://www.keytlaw.com/leasinglaw/azevictions.htm

The best way to deliver the notice is to hand it directly to the tenant. You may also deliver the notice to a person of suitable age residing at or in charge of the leased premises. Handing the five day notice to the tenant's five year old daughter would not be acceptable, but handing it to his twelve year old daughter might be. Sometimes tenants won't come to the door when you knock. It is acceptable to post the notice conspicuously on the front door of the residence if you also send the notice by certified mail to the tenant the same day you post the notice.

Assuming she gets a certified letter in a day or so, it would appear to be legal service.

The server will turn it into the court showing that the served it according to your state law.

So you will have to show up in court to argue the serive or you basically lose by default if you don't show up