"I spoke to your mom and the let me borrow her key"? What a wingman mother.
"It stands to reason that a girl's not gonna let herself into some guy's apartment if she doesn't like him"? She seems to know her hentai situations quite well. And personally, I don't think he was exactly that slow on the uptake, if anything, I think he got the message a lot sooner than most would.
Great ending though. Well hey, this is a lot more important than something like work, I'm glad they have their priorities straight.
And, wherever she may be, Senpai's mother sits contentedly sipping some drink, thinking with a smile: "Mmm, ... I feel I'll be seeing my grandkids one fine day."
Ending up banging so much you're late for work the next day after reuniting with someone you don't want to let go of... you know, I'm failing to see a downside here. Though I do hope the panties came off fast. Chaffing you dick will limit you ability to satisfy you cute kohai.