SgtLumpy is right.
There is a major diffrence between using brute force and calculating the leverge.
Woman tend to be a little more adapt at it since we come into such issues much more often (ever wonder how single women open the pickle jar?) Where there is the will there is almost always a way.
The trick with firearms is it's almost always about leverge. In all my years fiddling with guns I've only found one thing that requires me to use brute force and it's not anything to do with slides.
Being timid is usually the fail move in a lot of activities, and with guns as well. Trying to move a semi auto into battery slowly just doesn't work. The action requires the full force of that slide going forward. Trying to move it slowly will make you more likely to ride it a second as you let go. It's not a physical action that requires a lot of strength, but it does require decisiveness.
It's stupid easy and if you try to make it less stupid easy, you screw it up. Overthink it, and you'll never get it to work.
fragtagninja, another thing for both you and the wife to keep in mind is don't push it to the point she becomes frustrated with it.
Try the diffrent things suggested here and if they don't work after a few goes at it, just let it sit for awhile. Do something else, relax and have some fun. You mention your dad has some other guns? See if she can't shoot those for a bit and just set the Sig aside for awhile.
Come back to it later, when she's relaxed and in a more confident state of mind.
Or, if she wants to push it and gets in that PO'd "I'm gonna freakin' do this already" mood, just leave her alone and let her figure it out.
The above are my two main go to moves when I can't get something to work, and in the end I ALWAYS get it to work.