I gave this a try:
Find a Grave lists 4 Ruth Jones buried in Cumberland County, Main. However there are large number of Jones burials in that county. And I don't think they are who we are looking for.
There are only 9 Fosdick burials in that county, and I don't see a "Frank" listed
There are two Rufus Horton Jones listed in Evergreen Cemetery in Portland Maine (one is Rev. Sr. 1877-1907 and the other is Jr. 1907-1976 (that's weird) unless the dad died the same year the son was born.
Ruth could have been cremated. Ruth could be buried without a head stone (but the cemetery should have records of a plot)
Or the Find A Grave taphophiles missed her marker
Or maybe she went to live with a son or daughter and is buried in that area. This happened a lot in my family, where the husband was buried in Nebraska and the wife, never remaried was buried in California, because she went to live with her children.
I have not checked Family Search yet.
Good mystery! Keep searching!
EDIT: Find A Grave does not have a listing for her in Lincoln County, Main. That only means those cemeteries are not fully documented.