I don't really know how friend codes work for the friend safari. I know that you need to be friends with someone for it to work, and the Pokemon and Pokemon type depend on their friend code. I have a lot of friends (22 I think), but only 2 of them had ground Pokemon, and neither of them had Diggersby in it :(
Later on I found out there are a slew of websites dedicated to trading friend codes for the friend safari, and I found a guy who had the ground type with Diggersby. He displayed his name and friend code, so I added him on my 3DS friend list. At first I thought it would be all good and dandy, that I'd have my own huge power Diggersby in no time, but sadly he didn't appear on my friends list in Pokemon Y, or the safari zone.
Am I doing something wrong? 'cause I REALLY want that Diggersby tho.