I think the only option for a person to be marked as widow/widower is to create a couple and mark one of the partners as "Deceased". If you click on the button "Partners" (between "Personal", "Contact" and "Biography"), you'll see a list of all partners a person has had. After clicking on "Edit details", the website offers you different kind of specifications for the relationship ("Married", "Annulled", "Reconciled" etc.) and also "Other" which allows you to create an own label (that could be "Widowed").
Maybe there is another option, but if, I'm not aware of it.
Have a nice day!