I suppose not everyone is particularly mature in their early 20s. The dads girlfriend could be younger than he is. I am thinking some kind of 'aquaintance' who they may have been familiar with for one reason or another ( parties and other stuff?) if not a friend in the true sense. There are people who have very volatile relationships also, where they end up turning on people quite viciously.
I have tried to avoid what I consider an elephant in the room. I have a hunch that the relationship is built around 'other stuff'. For an island of 6,500 souls, it seems rather rife from the Scottish Government stats I had a look at.