Given in the tabletop you cast the spell on yourself rather than a target, when you look down the stairs to see an argument going on, you should be able to take the reasonable action of casting Friends before getting in the conversation.
Technically, yes, you cast it on yourself. But that's only because the wording of the spell is terrible. It gives you advantage against one creature of your choice, and you have to choose that creature when you cast the spell. From a programming standpoint, it makes absolute sense that you cast it on a different target. But you should be able to cast it mid-conversation; perhaps a Sleight of Hand check or similar to do so undetected.
