On Friends, Ross's girlfriend who shaved her head was Bonnie. Played by Christine Taylor, Bonnie is introduced as one of Phoebe's friends and becomes Ross Geller's girlfriend after Ross and Rachel's first breakup.
Bonnie had previously kept her head shaved, but by the time she met Ross, her hair had grown back. Rachel becomes jealous of Ross's new relationship and manipulates Bonnie into shaving her head again by pretending that the look is attractive and practical. Ross is visibly uncomfortable when Bonnie returns with a shaved head, creating one of the episode's most memorable comic moments.
The storyline occurs in “The One at the Beach,” the Season 3 finale, which originally aired in 1997. A common mix-up is Phoebe's earlier mention of another bald friend named Abby; Abby and Bonnie are separate characters. Bonnie is also sometimes remembered simply as “the bald girlfriend,” but her name is explicitly Bonnie.