Actually, you only need 25-30% of the blue bar to unlock the girl's place.
For the bad friend in the Relationship, it's kinda weird.
There are 3 ways I found so far:
1 - Get the red bar filled.
2 - Get the Hobby/ Charm to Level 5.
3 - As you said, have the one with one Bad Friend in their Relationship List to communicate with other girls. (like a virus
)
To decrease the Red Gauge, relaxing at their place, such as Warm up or Take a Nap in the Bathtub, for instance.
So far, I only see my character in Fatigue state more often, don't know much about interactions as the Relationship Term rises so fast in this game that it's hard to notice the differences.
When it's reach MAX, you will the lose the character data.