Bottom line of if all: it doesn't matter what anyone else thinks is fair or reasonable, if Leigh is uncomfortable with the situation and he is, it's his house and with no lease in place he can effectively do what he wants to rectify the situation. Money will make him feel more comfortable so he can rightly ask for that.
As for mates and holding grudges, well Leigh's room mate changed the agreement here. He never said anything about a girlfriend living there when they made the agreement and anyone who thinks the increase in cost of living is so marginal it doesn't matter probably hasn't lived in a share house before - it makes a difference. Regardless of cost of living, just having another person in the house with their stuff etc. reduces living space, interferes with television watching etc., ability for Leigh to invite people over before it gets crowded, dirt in house etc. It's a person in the house that Leigh doesn't necessarily trust just because his mate does. She could steal shit for all he knows. There are heaps of variables it changes having another person.
Also, see how kind Greg is with a 10 strong Filipino family running around the house and how much patience you have after a couple months! There's a reason hotels have guest limits.