The 'Vogue' singer has previously worked with the Fashion Police host on her Material Girl clothing range and has recently been giving her tips on how to lose the weight she put on earlier this year.
An insider told Closer: "Madonna's helped Kelly get back to a size eight. Kelly does the Ballet Barre workout five days a week - she loves the dance element and can see how it's changed her shape.
"Madonna's also encouraged her to eat in a more macrobiotic way - cutting out wheat, artificial sweeteners and cheese.
"Kelly's avoiding carbs to beat bloating and has been eating boiled eggs for breakfast, grilled chicken salad for lunch and fish with steamed greens for dinner."
Kelly is not the only member of the Osbourne family to take on a new dieting regime. Her mother Sharon has recently started the low-carbohydrate Atkins diet.
Sharon has said: "I've struggled with my weight for years, and I wanted a diet plan that would ensure I'd look and feel great.
"After looking at other weight-loss plans, I realised that Atkins is the best plan for me because my body - like many women my age - doesn't process sugar and refined carbs well."