Look up the deed in the county records. Download a copy if you can. It will be important for Mary to know how the house is titled on the deed. Not think she knows but actually see it on the deed.
Ideally, it should read Joint Tenants with Right of Survivorship. If it does, then Mary automatically owned the home at the moment of John's death
If the deed just says John and Mary then the statutory presumption is that the property is owned as Tenants in Common which means, in the absence of a will naming Mary, John's interest would go to his children and Mary would be partners with John's children in the ownership of the home.
The above information was obtained with a random internet search so it would behoove Mary to consult an attorney to verify the comments.