Can a woman commit rape?

Of course.

The Mantra of the BDSM community is safe, sane, CONSENSUAL.

Read that last line again!

CONSENSUAL That said, it is in the news today that Melanie Martinez, a musician ALLEGEDLY raped a long time friend, also female, with a sex toy.

From Wikipedia: Assault by forcible stimulation of female genitalia by a female perpetrator is possible by digital manipulation, oral sex, strap-ons, other dildos or other foreign objects, or tribadism. A telephone survey conducted in 2010 for the U.S. Centers for Disease Control and Prevention found that 43.8% of lesbians reported having been raped, physically abused or stalked at some point by an intimate partner; of these, 67.4% reported the perpetrator or perpetrators as being exclusively female. In the same survey, approximately 1 in 8 lesbians (13.1%) reported having been raped in their lifetime, but the sex of their rapists was not reported.
A 2005 survey by the California Coalition Against Sexual Assault (CALCASA) concluded that one in three lesbian-identified participants had been sexually assaulted by a woman. Fear of coming out, distrust of and/or indifference and hostility by the police, unwillingness to turn in members of the LGBT community, and a sense that female on female rape is not taken as seriously as rape by men causes some victims to be reluctant to report rapes by other women. In one publicized case involving students at Smith College in 2005, two women were charged with the rape of another, but the victim ultimately refused to testify, and charges were dropped.

That said, regardless that she's female and a known musician -- she shouldn't get a pass if she's guilty.