This might be a dumb question but..
if your not comfortable with her carrying a gun why are you?
Also why a butterfly knife? Why not an assisted knife or a quick flip knife?
If your not worried about her getting close to the perp (Spray dosnt work on everyone) and your looking for a stun gun/Tazer then if the perps holding her from what I understand she is ultimately shocking herself as well, with a knife if you dont see her able to pull the triggle she might not be able to cut someone enough to get them off.
Proper training would be your best bet.
1. I carry because I have training and in such event I would be capable of using the firearm if needed. It's not that I'm not "comfortable" with guns, it's that I'm not comfortable with people carrying if they are not willing to use them when/if the time comes. If she felt she could use it if she had to I would give her one of mine to carry.
2. I'm not sure why, I just like butterfly knives. I am very good with them and can open it faster than I can any other knife. I have plenty of knives already, but I like to add to my collection, not to mention they are fun :D
3. The knife isn't for her, it's for me, and yes you are right, pulling the trigger is a lot easier than having to use a knife in SD. The stun gun would have been for her. I looked into them a lot and can't find a decent stun gun anywhere so that's out of the question.
If the butterfly knives for you bear and son makes a great looking damscus one, and a new company benchmark makes a nice on on the cheap as well
Knives at Knife Center
this is a nice lil stun gun that she can carry in a purse or pocket and not worry about it going off on her reaching for it
Cheetah Matrix Self Defense Stun Gun - BUDK
Collectible Knives and Swords
hope this helps some
Just a note on Tasers, yes they are a bit expensive but they are very effective. If one of the barbs miss the target and they get to contact distance you can use the taser as a stun gun. The advantage is that if the barb that struck home is still attached the circuit is completed and you get the desired muscular seizure. If both have missed then it acts as a standard stun gun, applied to the groin, throat or other sensitive areas they are still effective. The other thing is a LEO Taser fires for 10-15 seconds but the civilian version fires for 30 seconds so your wife has time to leave the danger. The company recommends that once the BG is flopping on the ground just set the Taser on the ground and run like heck. Taser will replace for free a Taser that is lost in a defensive situation. Just file a police report and submit it and they will send you a new one. Just some thoughts.
Since you said you have some training in martial arts, you already know that the one constant of any fight is that you need to be prepared to get hurt in your defense, as a possibility. A knife fight is fast ugly, bloody and painful. Been there.....don't want to do it again. I only mention this because your wife will need to confront this fact, if she is planning to carry a balisong. Enough said on that topic.
Now to the stun gun. We all know that a stun gun is very different from a taser. Since you specified stun guns, here is the truth on them. (I have trained with them and been stunned as part of that training, many times.) They are a pain compliance tool. In practice, they do not incapacitate the target subject. Let me repeat: NO incapacitation. No falling down, twitching, drooling, or instant unconsciousness with a stun gun. It simply hurts enough that they may lose focus for long enough to be controlled and cuffed. Or maybe you could run away. but.....and it is a huge but......using a stun gun can also simply piss the subject off. Not good, since at that time, they are likely to be right in your face.
So, let's review: A ballisong is a very advanced tool to be used best by a very experienced operator, and perhaps as a last resort. A stun gun is a mostly ineffective incapacitation tool, that could infuriate an attacker, rather than turn them away from the attack.
My humble advice would be to use the space that a stun gun might take up to carry more extra magazines for your primary gun, or perhaps to add a good flashlight. As far as a knife is concerned, by all means carry as many as you or your wife choose to. But PRACTICE.......PRACTICE......PRACTICE!!!!
I hope this is of some help.
I was thinking of this and my ol lady works at sears now and isnt allowed to carry a knife on duty. and we dont own any firearms yet but I saw this the other day that caught my attention.
for the small 2AA Flashlights
Amazon.com: Bust-A-Cap BAC 60000 Tactical Tailcap for Maglite Mini Mag AA Flashlight: Sports & Outdoors
would be a good backup tool as well. and its a mad light so its bright (new ones are LED)
with a good small knife, would be a good combo