Coulson and Frigga go for drinks at least once a month. Not coffee or tea, but proper drinks. They usually meet at some dive bar and swap stories. The only person who knows is Darcy because one time, Frigga ended a bar fight with a few drunken brutes … that Coulson may or may not have started. 

Darcy just happened to be the most recent number in Coulson’s phone and Frigga had to call someone to pick them up and help deal with a passed-out Coulson.