I believe that the argument over rent money comes from Rudy.
"But the American girl was spending her time with far too many peculiar people, in the opinion of her three housemates, who didnt appreciate running into these conquests at breakfast. Meredith seems to have been the most put off by Amandas active social life.
...
Meredith told her tight circle of British friends, Amanda had acquired several boyfriends in succession.
This was by no means Merediths only complaint. There are rumors of anger over the rent money, and she found Amanda sloppy about her personal habits. Merediths close friends also found Amanda a bit odd. Amandas behavior always struck me as strange, one of these friends would later tell police. The first time I met her we were eating in a restaurant, when all of a sudden she began to sing in a loud voice. It was very strange and out of place.
http://www.vanityfair.com/culture/features/2008/06/perugia200806