So the blond girl, Asami, big part of her character is that she worries about Sayu. Presumably she's been in a similar position before or knows people who have. She immediately assumed (like everyone else) that Yoshida would have bad intentions, and invited herself over to confirm that the place Sayu is staying at is actually safe. Her being protective over Sayu (or vulnerable people in general, she barely knew the girl after all) is a big part of her character.
Anywho, new guy appears, confirms with Asami that he's bad news by telling her that he wants to take in a runaway girl and has been dating seven girls simultaneously. Asami, in response, tells him that there's a new girl, who's also really cute. This completely flies in the face of the character she was before. Like, why would she willingly put Sayu at risk like that? The "but if you do anything to her, you're dead." is promptly ignored by the guy because of course it is.
Are things so dire now that we need to rely on:
- coincidences (the guy happens to be working at the same place as Sayu whom he was sheltering before),
- absurd behaviour (how would new guy be able to date seven people at once? There's only so many days in a week. I get he's an extrovert, but you still need time to eat & sleep. Two-timing would have been good enough to get the point across),
- and characters behaving completely opposite to their intentions (wtf is Asami doing?)
to drive the plot forward?