Girls & Panzer dj - Miho-san no Otomodachi

Itsumi Erika visits Oarai after being invited by Miho. However, Erika drops in unannounced and thus runs into Shimada Arisu, who already had prior plans with Miho. Raising into question the nature of Miho and Erika's relationship. Is Erika Miho's friend, or not?