Another option is a Tanker style rig for your wife.
Here is one from El Paso Saddlery.
If I was worried about drawing extremely quickly I would make sure I used the bottom strap that is designed to be secured by your belt. That way the holster remains in place while you draw the handgun quickly.
I tend to wear mine over insulating layers but under my Goretex when hunting in very cold weather and it has worked well over the years. The main one I use is designed for large revolvers and is quite comfortable. I think it may be a good choice for the wife as it would allow her access and a wide range of adjustment to fit her body type.