The Lady is a Cavewoman

(from ebookjapan):

Neely, a college girl, went to her grandfather's cottage with her friend Killy during their summer vacation, but the cottage that used to be there was gone, and her grandfather appeared in the form of a caveman.

Neely sympathizes with Grandpa, who has built an atomic hometown where people live in the wilderness without the use of man-made structures, but Killy leaves him out of love.

Then Ryan, a male friend who joins them later, comes to live with Neeley and Grandpa...!

In addition to "The Lady is a Cavewoman", the collection also includes "I'm Learning To Tell Fortunes", "Black Cat Tango", "My Sister Marin", and "There's Room for Rent".