Hello,
This happens when there are no mines available for you to steal. This is an NPC generated to allow you to have a mine to steal so that you are not prevented from using the mine plundering option. If you check your log, you should see their name come up as "Mysterious Ninja." You should also notice that the mine level is Level 5, but that no other mines are unlocked, which is impossible for players.

If you'd like to try to match with your friend, try attacking at another time of day.
Thanks!