A friend of Dougie's told the Metro newspaper: "He is devastated. He didn't see it coming at all and still doesn't know why. They were planning to settle down and were actually house-hunting.
"It's the second time Frankie has done this after she dumped him in February but came back in tears and said she made a mistake."
The friend said: "Frankie always seems to be out partying, playing the celeb scene. The McFly lads don't really do that."
Although the couple battled to save their relationship after reconciling earlier this year, Frankie realised things weren't working out.
The source said: "Frankie felt they had gone as far as they could. They split earlier this year but got back together to make things work but sadly it didn't. Frankie has moved out of the place they shared."
Dougie - who had dated Frankie for almost two years before she dumped him last week - recently confessed he was waiting for her to propose.
He said: "I'm waiting for her to ask me. We're new school. We wait for the women to ask us. I'll have a big diamond please!"