The failure on a friend's part to step in and prevent a terrible situation from happening. In most instances, the friend not only fails to prevent the situation from happening, but makes an observable effort not to help. This is an allusion to The Office Episode where Jim leans back and allows Michael Scott to fall into a koi pond.
Dude I can't believe you didn't do anything back there. Way to let me fall into the koi pond, "friend"