I have a friend who carries a knife for protection. Anyone know the statistics on them being used against the owner in a defense situation? I want her to know how much more dangerous it is.

This site is from the UK but it states that 65% of people who carry knives have them used against them- http://www.insight-security.com/facts-knife-crime-stats.htm#Statistics%20-%20the%20numbers

Maybe you could take your friend to a self-defence class, it would be a fun thing to do together and it would make her feel safe enough to not carry a knife.

I can appreciate the fact that you care for your friend but I can't agree that a person should not have something to defend themselves just because it's possible it could be used against them. You never know how far an attacker will go. Some attackers don't intend to kill you and some do, but you have no way of knowing the difference, therefore you have to assume the worst in any case. If the attacker intends to kill you then it really doesn't matter if they do it with your knife or with their own. But by having something atleast you have a better chance of getting away. I agree that it's possible an attacker may use your knife against you, it carries a certain risk. But like I said if an attacker has a knife of their own what does it matter which knife he uses on you....in that situation having a knife unknown to the would-be killer may buy you enough seconds to surprise them with it and save your life.

There is a point in favor of carrying the knife.
She knows that she has it. This makes her more confident. A person who appears to be self-confident has a lower risk of being attacked. It is like having a widely armed society...the creeps don't know who has a weapon and who might blow their goodies all over the wall. They are less apt to mug old ladies if the old ladies might plunge a knife into their soft spots.
As far as any stats from the U.K., they have higher knife crime than the U.S. does. My understanding is that it is a lot higher. I don't think it can be compared to any situations outside of the U.K.
I agree with H.S. (I don't like that name!) If you are attacked, who's knife does the damage won't matter. Maybe, if she had a knife, she could score a good notch on him before he killed her.
In any event, it is our responsibility to protect ourselves. We do the best we can. And a knife can come in very handy in lots of situations!

take her to a self-defense class. carrying around a concealed is a good idea, but having training is safter in the long run.

Tell her to get self defense training instead of trying to take the knife away from her. Better yet, get her trained to use and equipped with a gun. The problem with knives is that you do have to get within the reach of your assailant in order to use one. Guns on the other hand allow the person to keep potential assailants at a distance. Statistics show that merely drawing a gun out of concealment goes a long way to stopping criminals. However, with any weapon, the person using it should have proper training so that they know how to use it. Proper training would also reduce the odds of someone being able to take it away.

When I was in the Army I used to teach a self defense class. It was practical self defense for women. Women (I do not care what anyone says) are weaker than men in a physical altercation. My class used to teach two things. Mindset and using whaterver weapons were available. Cans, silverware, golf clubs, baseball bats and so forth. The hardest part of teaching this class was the mindset part. At the start of the class I would hand a woman a bat and my armored assistant would come at her. The strikes the woman landed on my assistant were always tentative and light. He would walk right through them and put the woman down. The women were al squeamish about hurting the attacker.

People have to realize that when you are attacked, the person attacking you has every intention of hurting or killing you. It is not something they have to think about, it is something they are already committed to. To properly defend yourself you have to have the intent. If you carry a knife for self defense you have to be willing to punch it deep into someone's guts. You have to be willing to slash someone deep enough to cut to the bone. If your friend can't do this then carrying a knife is a greater danger to her than to whoever may assault her.