@lly to be fair, he didn't say it was a girl. He said it was a friend.
Still there's tons of issues with that entire situation ranging from "why are you accepting a random old strangers invitation to take a shower" to "how can you just bust into someone changing in the bathroom like that" to "how the heck is grandpa leaving after inviting a complete stranger into his house". So many questions that go beyond the "it's Japan" explanation.
Well, the Gramps thought that she was Wakana's friend. Hence why Gramps said that Wakana's friend visited. Sadly for the Grandpa, Wakana only has one girl-friend.
No love triangle please, spice things up sure but the main couple is already decided so don't be dramatic about it. Well, Kitagawa's just too cute though, I don't know if any other girls would match her.
@Feeder It's quite strange. Manga teaches us that nobody in Japan ever knocks, they just go forward to open the door, no matter the door or what might be behind it, yet the doors are never locked. Except for the school PE equipment storeroom.