David, the fact that you even bring it up for discussion shows that you have a conscience, and that you care about the privacy of the person that you bought the camera from. Nobody is perfect, and I don't think what you did was substantially wrong. If you have her address just send the film back. If not, and she doesn't reply within a year or so, just discard it.
Would it have been better to hand her the unprocessed film? Sure. But you didn't, and it's not the end of the world. You offered to send the film back to her, and she didn't respond. That says to me she isn't that interested after all. If you need to explain yourself, it's probably best that you thought it would be a nice thing to do, to process the film for her, but on second thought you regret doing so. If she can't find it in her heart to forgive you for processing her roll of film for free, and offering to send it back, then I think she's the one that has a problem, and not you.