Two adults (friends, nothing else) own a house and live in it. They rent the spare room to their friend.
Friend pays less than market rent for renting the room, It's a small room, the house is a tip from being a work in progress.. Just being fair.
Should all three split bills equally? I think so, but others don't. Arguments against are things like 'I'm always in my room!' 'I was here first!' 'You pay less than anyone else'! etc etc.