I pretended to be a manufacturing engineer last summer.
One of the particularly illuminating ideas in manufacturing engineering, and one that can be applied to all sorts of things in the rest of life, is that if a worker isn't doing something the way you want them to, often it's that the situation isn't facilitating it. So I'm reading through this thread and people have a lot of great suggestions about improving technique, etc. but they all rely on getting the OP's wife to do something that right now she doesn't want to do. Or at least, no longer wants to do if she's tired or something else is going on.
I think that the posts questioning the suspension setup are right on. If she's descending with her ass planted or has a lot of weight on her hands, rather than telling her to fix that - I'm sure the OP has - I think it also makes sense to look at why it's difficult for her to ride correctly.
Something I hate about these threads is that I do think individuals need to take ownership over their own bike setup. It has helped me to get one of my bikes fit by another person. But ultimately, I fit all of my bikes, and I fit them to my attack position. I think husbands trying to fit wives' bikes are a recipe for a sitcom, and probably not that good for wives having bikes that fit well. Do women not know how to use the internet? I guess forum gender statistics would support that idea...
I don't have to fight my bike to ride in the attack position. Instead, the bike and I work very well together when I take my attack position, and it's easy for me to be "good" about making my platform, having quiet hands, driving the bike from the hip, etc. OP, if it's hard for your wife to ride in the attack position and she knows what it is, find out why it's hard for her. Is the bike set up wrong? IME, landing the grips in the right spot is huge. My pedantic coworker last summer was big on the "Five Why's." You ask someone why they're doing something a certain way, from a position of true curiosity and without judgement (good luck!) and then follow up on the answer until you've asked "why," five times. For example, you might ask someone why she doesn't like to get out of the saddle, then why the cause of that is occurring, etc. I'm more inclined to try some things and see what sticks - I've tried a couple stem sizes, different positions for the stem, etc, and am quite happy with where I put my hands now.
My bike doesn't nosedive when I brake or roll something a little less-than-elegantly. So, I don't want to lock out my fork. Which is good, because it doesn't have a lockout.
OP, I wonder if your wife's fork has too little pressure or too little compression damping. Too much rebound damping is a possibility too. Even at 160 lb, I ride with almost no rebound damping - you really only want enough for the fork not to kick, or maybe one click more if that mellows things out a little. IIRC, the recommended order for setting up this stuff is to get sag first, then rebound damping, then compression damping. I know you say the fork is set up for your wife's size, but it's clearly not working for her. This stuff is iterative, too - I think I can get a fork "pretty good" on a first pass, but it takes some riding and messing around to get it really dialed.
All of that adds up to it being fairly easy for me to ride my bike down a hill in the way I perceive to be good technique - pedals at 9 and 3, ass off the saddle, and quiet hands. It feels like skiing to me.
I'm still working on leading with the correct pedal all the time, but I'm very right-dominant and I don't think there's anything I can do with bike setup to fix that.