I think its an old superstition. Most people these days dont follow it, but some people still have that superstitious nature, that if a widow or divorced woman applies mehndi to your hands, the same fate will befall you. And I think people who are widowed/divorced sometimes act upon it more, because they dont know how the people hosting the mehndi feel. My family doesnt believe in such things. My parents best friend is a widow, and they have asked her to apply mehndi my brothers wife when it was their wedding, and to my hands as well at my mehndi. AH, we are all happily married. Allah has written your kismat, and a widow applying mehndi to your hands is not going to change that.