Just because she has "100's" of books, does not mean they are books you want to read. Plus there are free books everywhere, more for your sony (without converting) that for the Kindle....
Your question is "what would you do".... I would strip the DRM off the books she has that I want to read, and read them on my Sony. When I was through reading them, I would delete them. Therefore making it like she had loaned me her old school paper book. That is what I would do.
What you want to do on the other hand, is up to you.
If you like the Kindle, then you should get it. Then if it is to be your reader of choice sell the Sony now while it is worth something.