The star has tweeted at length for the first time after a reported overdose earlier this year.Read more
The response comes after a fan named Angela described the move as "rotten", suggesting Lovato's team had treated Martin badly after she gave an interview shortly after Demi was taken in for treatment.
“It was completely unfair to fire Dani, she’s one of the few who really cares about Demi’s health and has been helping her for three years," she wrote on Instagram.
"She [Demi] is surrounded by people who care only about her money and she does not see it."
She added: “The Demi team is so rotten and she does not realise it, needs a total change..."
Demi, who only recently returned to social media - urging people to vote in the midterms with a voting booth pic, replied succinctly: "You have no idea what you are talking about.
"True friends don't do interviews when you OD".
BBC Radio 1
He covered Demi's Sorry Not Sorry and you can check it out above.
This follows Demi doing his track Take Me To Chuch, in the same room a few years back.
He Tweeted: "Was a pleasure to perform a cover of Demi Lovato's Sorry Not Sorry for Radio 1's Live Lounge and return the love for her awesome cover of TMTC."
Demi recently broke her social media silence after a period recovering from an overdose.
Next up is Zara Larsson on Monday and we cannot wait.
The Democrats took control of the US House of Representatives in Tuesday's mid-term elections, dealing a blow to Republican President Donald Trump.
This news will have been music to the ears of the many high profile singers who came out in support of Democrat candidates.
Including Beyonce - a registered voter in her home state of Texas - who showed her support for Beto O'Rourke.
Sadly for Bey, O'Rourke ultimately lost to Republican Ted Cruz.
Prior to the results, Bey posted the above pic of her wearing a Beto cap, underlining the importance of voting to her 119m Instagram followers.
"I’m feeling grateful for everyone before me who fought so hard to give us all the right to have a voice," she wrote.
We can’t voice our frustrations and complain about what’s wrong without voting and exercising our power to make it right. We need you. We all need each other, because when we are truly united we are unstoppable.
"Sending you all love and positivity on this happy voting day! Every vote counts, Every race matters. Everywhere."
Demi Lovato broke her lengthy social media silence to post a quick pic of herself in the voting booth (below).
The singer, who has been recovering from an overdose, wrote: "I am so grateful to be home in time to vote!
"One vote can make a difference, so make sure your voice is heard! now go out and #VOTE!!!!"
Pop stars have been particularly 'woke' during these elections, with Taylor Swift publicly nailing her colours to the Democratic Party mast for the first ever time.
Last week, Kanye West distanced himself from 'Blexit' T-shirts, which were encouraging young black people to turn their back on the party.
And yesterday Rihanna became the latest to send Trump a cease and desist letter to stop using her music for his political rallies.
Kendrick Lamar was the big winner, while the mother of Heather Heyer received a standing ovation.
Demi Lovato turned 26 yesterday, and the pop star's family have taken to social media to share some heartfelt thoughts.
Just a few weeks ago, Lovato was admitted to hospital after a drug overdose, and is now receiving treatment in a rehab facility.
Lovato's sister, Madison De La Garza, shared a heartfelt message on her Instagram page, thanking the teams who saved her sister's life.
"Today could have been one of the worst days of my life, but instead i get to spend it thanking God, the incredible doctors, Demi's team, and everyone across the world who offered their support - without all of these people i wouldn't have my big sister anymore.
I've been thinking about how i wish that everyone could see the silly little things that she does, like how her nostrils move when she says certain words and when she brushes my hair behind my ear when i'm trying to sleep, because those are the things i'm thankful for today. they seem so small, but those little things make up my sister - not a singer, not a celebrity, and definitely not what she's been described as in the media - she's a daughter, a friend, and my big sister... and i am so, so thankful that i can tell her happy birthday.#happybirthdaydemi❤️
It’s the first time Madison has spoken out after Demi's overdose, though she was among one of the first people spotted going to her house after she was rushed to hospital.
Demi's mother, Dianna De La Garza, also posted that she felt "blessed beyond belief."