'Miss' is for unmarried women, usually never married.
'Mrs' is for married women.
'Ms' simply denoted female.

However, it's rarely that straightforward. There is a misconception that 'Ms' equates with 'divorced' although divorced women can use it of course, but strictly speaking, any female from 9 months to 90 can call themselves 'Ms'. It doesn't denote any marital status, just gender, and is intended to equate to 'Mr' in men which simply indicates 'male.' Some people still think 'Ms' too 'women's lib' and plenty of folk misconstrue it still.

Very many women keep 'Mrs' after widowhood. 'Miss' can also be used as an honorary title, for instance, many female singers are called 'Miss' although they are married.

So a direct answer to your question would be that it would be fine to call her 'Mrs', OR 'Ms' depending on how up to date and socially aware of the finer points you think your characters are.