"I am horrified and saddened by the statements and story told by Timothy Heller," The Voice singer said.
"What she and I shared was a close friendship. We tried to help each other. We both had pain in dealing with our individual demons.
"She never said no to what we chose to do together. And although we parted ways, I am sending her love and light always."
The pair appear to have been friends since at least 2015, but Heller said their friendship came to an end because Martinez could not dedicate any time to her.
In the astonishing claims posted on social media, Heller wrote: "I have kept this secret for years, convincing myself that it wasn't a big deal and I wasn't hurt by it.
"The thought of accepting that my best friend raped me seems insane.
"One night during a sleepover, she became increasingly interested in my sexual preferences.
"As someone who had previously been through sexual abuse, sex is hard for me to talk about. I was obviously uncomfortable, but she was my best friend, so I tried to be open about it."
The accuser then said Martinez repeatedly asked her to have sex over the course of several hours.
Heller claims she said "no" over and over and eventually they fell asleep.
The following night, the same thing is said to have happened - this time Heller said it ended with her smoking cannabis and being groped and raped.
"I never said yes," Heller said.
"I said no, repeatedly. But she used her power over me, and broke me down. Just so there is no confusion, I was molested by my best friend... she performed oral sex on me and then I was penetrated with a sex toy without being asked.
"That's what happened."
Heller ended her story with the hope that in light of "recent events, people will believe me," and the reminder that "girls can rape girls [and] best friends can rape best friends."
Actress Abigail Breslin waded into the accusations, saying Martinez's response was not good enough.
She tweeted: "Melanie, this is a completely unacceptable response.
"Not saying no doesn't mean yes. But for the record, she did and has stressed that numerous times.
"Shame on you for this disgusting reply that contributes to the victim blaming culture we live in."