Heidi, 40, started dating Martin, 41, a few months after she filed for divorce from her husband Seal after several years of marriage.
"Martin was there for her during a challenging time in her life and it was something she will also be grateful for," said a close friend of the model.
"They are no longer working together," another source revealed. "Life is about timing. He was incredibly supportive and they were there for each other, but they're moving on now. They still have a great friendship."
Mum-of-four Heidi first confirmed her relationship with her bodyguard during an interview on Katie Couric's talk show in September 2012.
"He's been with our family for the last four years," she said. "He's cared for our entire family, mostly for our four childen, helped us tremendously.
"I trust him with my children's life. He's a great man… and we just got to know each other from a completely different side."
Heidi's divorce from singer Seal, 50, is yet to be finalised. The exes were spotted sharing a friendly kiss in October when they got together to celebrate daughter Lou's fourth birthday.