It may sound crazy, but I think if you get in some poison ivy/oak and you have a lot of negative thoughts about having a bad reaction, it will get your immune system over-active and you will have a stronger reaction to it. Once I realized this and worked on not worrying about having a flare-up, my reactions to it dropped a lot. Could be a coincidence; people's level of allergic reactions change throughout their lives. I have noticed this same principal seems to work with other things too though: headaches, weird body pains, upset stomach,...
Um, yeah, I'm going to say this is a coincidence. The following story is anecdotal, but then so is yours so I feel comfortable using it as a rebuttal.
I thought I was immune to poison ivy--and maybe I was, for a while; it's true that your level of sensitivity can change throughout your life, for reasons or no reasons. The first time I had a reaction, I was doing an archaeological excavation which was, unbeknownst to us, in an area covered in tiny little poison ivy plants, all growing in the shade of slightly larger plants, and completely unnoticed by any of us at the start of the dig. I started to break out after a week, and others followed, and it took us another three days to find the plants and figure out what it was. My reaction was so severe I ended up in the ER with shortness of breath; my arms and legs swelled up like mad; I was on steroids and forbidden to work for a week.
For all I knew, I was immune to poison ivy, and nobody even saw it before the reactions started, so how could I have been less free of fear and negative thoughts? Not trying to be a jerk, but allergic reactions can be dangerous and encouraging people to go into one with the belief that they can make it go away with their mind is just irresponsible.
For the OP, I will also suggest Tecnu. You can use it on your skin, but also to clean almost anything of the oil, up to and including tools, pets, and clothing. When I was cleaning all the stuff I brought on the dig, I soaked my clothes and backpack in Tecnu, then machine-washed, worked wonders.
For your wife, though, here's another thought: I picked up a poison ivy wash when I was having my severe reaction, and I used it every time I showered. I don't remember the brand, but I got it in the same grocery store aisle as the Tecnu and other such products. It washes off any oil, and also has menthol which feels great on a rash if you've already got one. It might be worth finding something similar for her to just use on a regular basis during poison ivy season. I even used it on my face and it doesn't wreck your skin or anything like that. If she's breaking out a lot, some regular treatment would probably help, but using rubbing alcohol or even Tecnu might be too harsh for every day.
Speaking of menthol, I kept a bottle of the green Gold Bond lotion with menthol next to my bed. It was the only thing that helped when I woke up in the middle of the night with crazy itching.
Incidentally, scratching itself doesn't technically spread the rash--by the time you've broken out, assuming you cleaned off the skin at some point and you're not still being exposed, any oil has been fully absorbed into your skin and isn't hiding out in the blisters--all that's in there is pus. It can appear to be spreading because your skin will absorb different amounts of oil at different rates, especially with prolonged exposure, so rashes will take longer to appear in some places than others. Scratching can make the rash take longer to heal, and cause scarring, but it doesn't spread the reaction.