Q: Find Friends App - Not locating Friend

http://support.apple.com/kb/TS4008

Not so obvious but typical Apple direct.

In the Find My Friends app, at the bottom is entry "Me"

Go there..

In that page there is a "report location from ... " and I'd bet you or your unfindable friend have the wrong device checked.

Choose this device

What bothers is there seems to be no obvious way to delete old equipment

Be sure you use a unique name for each piece of equipment so you can keep it all straight

You 'delete old equipment' by removing it from your iCloud account.

must not happen immediately, as it still shows on our accounts