What I don't understand is why the jealous girl's best friend went that far (and not, you know, the jealous girl herself). Maybe it's revealed in the later chapters.
By the way, the wasei-eigo ビッチ on page 5 is actually the guys calling Mii-chan promiscuous.
(EDIT: This was a long-winded way of agreeing with you.)
Google Translate says ビッチ --> Bitchi --> Bitch, which as I understand it means Slut in the Japanese usage instead of Mean Woman as we use it. I would assume it comes from Female Dog (in heat).
Apparently the first derogatory use of "bitch" in English matches the Japanese usage. Wikipedia says:
The earliest use of "bitch" specifically as a derogatory term for women dates to the 15th century.
[8][9] Its earliest slang meaning mainly referred to
sexual behavior, according to the English language historian Geoffrey Hughes:
[13]
The early applications were to a promiscuous or sensual woman, a metaphorical extension of the behavior of a bitch in heat. Herein lies the original point of the powerful insult
son of a bitch, found as
biche sone ca. 1330 in
Arthur and Merlin ... while in a spirited exchange in the
Chester Play (ca. 1400) a character demands: "Whom callest thou queine, skabde bitch?" ("Who are you calling a whore, you miserable bitch?").