This is typical of the scare stories surrounding asbestos, and surveyors covering their backsides. This is only a problem if/when the garage needs re-roofing. The stuff (asbestos bound into cement) is perfectly stable when left alone and a problem is created when it requires to be taken off, because it may break producing fine flakes or even dust. It is that expensive (without guessing what THAT has been quoted at) because the contractors have to be approved, the operators wear protective cloting and the sheets double-bagged for disposal to an (expensive) landfill site.
She may have to negotiate though, to sell the house.