A good friend of a good friend. Even if you tend to hang out in the same circles often, this person would only qualify as more of an acquaintance.
Not someone you would desire to hang out with alone. Either because you only talk to the person large group settings and never really directly bonded, or because it's someone with whom you share nothing in common (other than a similar friend) and it would be awkward and uncomfortable to spend time alone together.
Most often it is a significant other's best friend.
"Nah, I don't see Megan anymore ever since Dave moved away. She was only a friend-in-law anyway."