Stefan, as I´m staying mostly in Bon Accord...
Sometimes it happen that locals rent out rooms/apartments into their own house or have buildt a house with 2-4 apartrments, build only for rental for guests. They sometimes call it Guesthouse, but that must not mean it´s a real business..
I would call the Swedish lady first of all, if you have her address. If not, call the guy and ask for the address, than you might get some useful information here from peoples who know the area.
But I would for sure not choose a place by recommondation from someone I don´t know very well. Quite many locals have small apartments for rent, but the standard and the equiments can vary VERY much.