Ben Affleck and Jennifer Garner have this whole co-parenting thing down pat! The divorced couple reunite as they attend a birthday party for one of Sam's pals. We spotted Ben and Jen engaging in very cordial conversation, on Thursday, following his revealing conversation on Good Morning America when he revealed getting divorced was his biggest regret.

Ben's comments about Jennifer reportedly made her boyfriend John Miller very uncomfortable ... source reportedly told Us Weekly, "John is empathetic to Ben's struggles with sobriety, but he worries about how Jennifer has once again become the focus".

Meanwhile, Ben hasn't been shy about exploiting his divorce and his struggles with sobriety to promote his new flick The Way Back. Check out the trailer ...