Plus, she's never specified how much of a reduced rate. I am not from the area, but I just had a quick google, and it seems that around 800 - 900 pounds a month would be roughly the market rate for a 3 bedroom, renovated home (if you know better, please correct me). And then say Louise chose to instead rent it out for 700 pounds. That's about 200 pounds a month she would be 'covering'... that's essentially pocket change to her. 50 pounds a week? She probably spends that amount on Minstrels. Even if it's more than that, she would be able to absorb that 'loss' pretty easily.
I would like to say, I think it's nice of her to do this. To the family who is renting out that home, 200 pounds a month would make a massive difference, but I think it's a very effective, and very affordable, type of PR for Louise. Same with the donation of the book sales thing she's doing (I don't fully understand that one, but some people broke it down nicely in the previous thread about how it sounds a lot more impressive than it actually is). Again, I think this is a NICE thing to do, and she didn't have to do it, so good on her. But this is not an entirely alturistic thing. She may be dumb, but she's not stupid.