It's possible it's the same Sarah, whose first husband died. However, Hay, Robertson and Davidson are all common names in that part of Scotland, and first names were chosen from only a small number of first names in comparison to today. So it's just as possible that the marriage of Sarah Robertson was for a different Sarah. Unfortunately the majority of death records for that period have not survived, and the only way you can be more certain is to know when William Hay died.
William Davidson and Sarah Robertson are in the 1841 census at Hay Farm, Cruden - she age 50 and he age 70. By 1851 she'd been widowed, and is in the census as Sarah Davidson age 65 at South Hay Farm. Would be worth looking for a death certificate for her on ScotlandsPeople as she may have survived until after death certs were introduced in 1855. If she was married twice, her death certificate is likely to say so.
As it happens, one of ggg aunts Isabella Fraser married George Wallace in 1858, and he was the farmer at South Hay Farm in Cruden parish. I also have quite a few Davidsons from the Longside area on my family tree.
