I think as a holiday let it could be a good investment but it’s very poorly styled. To make it appealing it needs to be very well presented and the furniture needs to be in proportion to the size of the rooms. At the moment it looks like it’s furnished with an assortment of handmedowns from friends and family. That’s fair enough, it’s how most of us start out but if your ‘friend’ wants to generate interest in such a tiny space she needs to up her game.
If she were to spend a little (under £1000) on a small sofa, a few floating shelves, matching baskets or boxes for storage and some mirrors, pictures, plants etc it could look very appealing and she’d still have all the stuff after it was sold. Hell, if she pays my train fare to Devon I’ll come down and do it for her - I love a project!