Mourning for dummies
There's no reason to offer your condolences to anyone who refers to themselves as a Strohwitwe: a "straw widow" probably won't be alone for very long. The term refers to a woman whose partner is away for an undefined period of time. A Strohwitwe has been left to sleep alone in their shared bed - which nowadays is hopefully made out of something more comfortable than straw.
The origin of the term can probably be traced to the Middle Ages. A woman who was unmarried but no longer a virgin was referred to as a Strohbraut (a straw bride). Despite its rather archaic roots, the word also has a male equivalent: Strohwitwer.
How long your partner is away before you feel widowed is rather subjective: some might become a Strohwitwe or a Strohwitwer after a few weeks - for others mere minutes might be enough.