"Old Friend" is what I call someone when I can't remember their name. It's a little social trick I've been using since the ninth grade. "Hello, old friend, it's so good to see you once more." "I admire your turtleneck, old friend." "What is your Donkey Kong high score, old friend?" It really works well, and helps avoid any social awkwardness.
However, in Mary's case I get the strong impression that this is simply code for, "We're keeping this strictly platonic, buster." We should hear Frank referred to as "old friend" at least twenty times. Today was the fifth time so far.