Regarding the rental: Sarah offered you a room at a price, and you accepted. It's really not your business how she chooses to spend the income she makes as a landlady (i.e., whether she uses it to pay down the balance on her mortgage or buy trinkets in Tahiti). Your mistake was mixing friendship with business and thinking that one would somehow sweeten the other. Whether you had a written lease or not, this was a monetary arrangement pure and simple—at least to Sarah it was.
As for the Bill situation, the most generous reading here is that you started prattling on about how super-psyched you were to go out with Bill. Sarah, seeing your excitement and not wanting to upset you, kept the news of her own rendezvous with Bill under wraps. I doubt she was "getting off" on the secret of her own date so much as she found the subject uncomfortable—not least because she liked Bill, too. But I'm curious as to why you say that if you knew Sarah had liked the guy, you wouldn't have gone on a date with him. Is that really true? Or are you saying that you expected her to have canceled her movie/dinner plans when she heard about yours?
In any case, since you don't mention a single positive quality of Sarah's, you ought let the friendship die a second and final death. I can tell by your use of quotes around "great favor" that you believe if she were a real friend, she would have offered you a place in her house for free. In short, you've answered your own question.
Friend or Foe
Dear Friend or Foe,
Four years ago, when I was a senior in high school, I started dating "Chris," the brother of my friend "Matt." It turned into a physically abusive relationship in which I felt trapped. All of my closest friends—including my best friend since preschool, "Tina"—knew I was unhappy. But they didn't know about the abuse. I ended up being hospitalized for three days. Once I was discharged, I immediately ended things with Chris and told my closest friends, including Tina and Matt, what had happened.
Matt said his brother would never do that, and the revelation ended our friendship. Tina was very supportive and said all the things a good friend would say. But that support lasted a month. I found out that Tina had decided to be friends with Chris again, and Chris started hanging out with other friends of mine, too. I felt compelled to pull away from all of them. Tina blamed the demise of our friendship on me, saying that I had changed. I haven't spoken to her or seen her or Matt in four years, but I still feel betrayed, hurt, pissed off, and sad. I've since made new friends and even have an awesome best friend. I also went to therapy to deal with the fallout from having been in an abusive relationship. But I can't seem to shake my bad feelings about Tina. Should I write her an e-mail? Do nothing?
Betrayed, Hurt, and Pissed
I'm sorry you had to experience such a traumatic series of events at such an early age. Life isn't the Mafia, but I think you were probably asking for the impossible when you expected Matt to turn against his own brother. Tina is a different story. I'm sure it felt like a second blow to watch your best friend since the Barbie-doll era effectively siding with Ken instead of you. If, four years later, you're still seething over the matter, I don't see why you shouldn't send her a letter telling her how betrayed you felt. You might point out that while everyone deserves a second shot, by inviting Chris back into the friendship fold almost immediately after learning that he'd abused you, she was effectively sanctioning his behavior. In that sense, she was a bad friend—not just to you but to all women.
Friend or Foe