There's really two answers to this question - one for now and one as you gain more experience. For now, if you're uncomfortable because of what you already know about the situation and you feel you know ahead of time what the cards will say, and you're not sure how to constructively tell her that - all of those are good reasons to do exactly as the reply above says and just tell her that you're too close to the situation to read objectively and she'd be best served by going to a reader she doesn't know.
But in time, and with practice, a couple of things should get easier. First of all, you may frequently find yourself in the position of reading for people you don't like. You may not even know them, but the way they act or the question they ask may give you a sense of dislike or unease. It's good to practice techniques for setting that aside. Remember that it actually doesn't matter how you feel about them - trust in the Universe to give them a good reading through you. You can acknowledge your own feelings or biases, imagine putting these feelings in a little box, and then closing the lid and setting it aside long enough to let the message flow through you unhindered. This allows you to give a professional reading to people no matter whether you like or dislike them, and is useful in a lot of other professions too.
Second, what will come with practice is learning how to give difficult news to someone. Let's say you didn't know her and saw what you did in the cards. I always believe the best approach is to be completely honest about what the reading shows, but delivered with as much compassion and constructive assistance as possible. Suppose you ask "what is the cause of your bad luck" and draw the Justice card. You can simply say - "Justice is associated with karma. It may be that you have wronged someone and need to set something right, and this bad luck is the Universe's way of bringing it to your attention. It may be helpful to think over the last few months and see if there is anything you might have inadvertently done that needs to be brought back into balance." This tells her 1) the bad news straightforwardly, 2) what she needs to do to fix it, and 3) allows her to save face by suggesting that it's possible it wasn't even intended.
Now if, because you know this person, she might believe you were speaking for yourself at that point, it would be best not to have done this reading at all - because even if it is what the cards show, there could be an appearance of bias on your part and she won't be able to accept the message as easily as she could from a stranger.
Hope that helps!