This term originated from a webcomic called penny arcade. Saftey monkey is one of the group's friends that they constantly abandon, make fun of, and "tell him to meet places and never show up"
In everyday conversation, safety monkey is used to refer to a friend that is just there to make fun of. Sometimes this friend can still be a close friend and only the constant taget for playful jokes, however he/she might also be seriously attacked by his/her 'friends'
"John is our safety monkey"
"He's not really our friend. just a safety monkey."