Actually, no. It's not strange that they were never arrested. It's not the usual route, I'll admit. Most people that are facing felony charges are in fact arrested at some point prior to charges being filed. Your friend could have been put on notice of the charges, gone in and arranged a release bond, then gone all the way through trial without having been formally arrested.
I agree with Mr. Dane that this is unusual, but very possible. Most folks get arrested for an offense, some do not. I have some clients who arrange "no-arrest" bonds so they dont get arrested. Basically, we find out about the charges, and set up a bond through a bonding company prior to arrest and they promise to appear in court, so no arrest is necessary.