I'm with alex on this, and I share a house.
You make up with it cause the guy living with you is a mate and having a roommate you don't really need to manage and isn't a complete dropkick is actually a large advantage you don't seem to realise yet
I'd say your mate has paid X for X service which he is receiving, his gf is piggybacking off what he is paying, not you, not really.
If her additional presence incurs additional costs (like water, or electricity, which really is all she'd be doing) then yeah, have some additional old bills before she came along ready because that's what she's "costing" you.
Living with people can turn to absolute shit pretty quick.
If this is the main issue it's not really that big a problem. Right up there with "My 400RWKW car kind of uses a fair bit of fuel"