Just my 2 cents:
1) Like most have pointed out, if you want her to fire in DA, then the decocker is what you're looking for.
2) Safety -wise...... my personal opinion (and I know this is something the gun community is split on) is that the manual safety is better because the trigger is blocked and will not pull. I'm used to handling M16s so I'm very used to that concept, but I do understand why other may disagree. Many consider decocking to be safe enough because of the long pull for DA and also allows for faster response in a situation where seconds matter (as opposed to fumbling with the safety switch). For me, I have it ingrained in my mind to flick the safety off and pull the trigger, so that's what I'm comfortable with. I would suggest you discuss both concepts with your wife and have her try both types in the range to see what she would prefer. Let us know what she ends up picking. Good luck!