Colt, I believe your wife may just feel embarrassed about masturbation. I know I did when I was younger. If my hubby asked(back then), I'd probably tell him no as well. Why?, I really don't have an answer to that one. I don't know if all females masturbate or not, I always have, can't remember the age I started, lol.
Anyway, during sex with my hubby, I'd get really into all the play(I was ok with him doing things to me, I just wouldn't), then one day, he wanted to watch me masturbate. I was so not comfortable with that. He assured me it was a major turn on for him so I went along with it. I remember he was using a toy on me and we started out with him placing his hands over mine as I held onto it. After he saw me getting into it, relaxing a bit, he removed his hands so it was just me. I hesitated just a bit, but in the end, it was exactly what I needed to open up completely, surrender all trust to him. That was many many moons ago and things just get better.
So, if you have any toys or even with your hands, place her hand over her clit and you move her hand over her. Let her get comfortable with both your hands down there for a while then let her loose. Tell her you want to watch, reassure her by saying things to her, play with yourself, etc. Only she knows if she's masturbated or not, so by going about it this way, maybe it will help her to understand that it's a good thing, that she should get involved in her own pleasure as well. Good luck
Added: A great position to try: her sitting between your legs, maybe on the bed, with both of you sitting up but also leaning back a bit. That way, you have easy access between her legs, especially if you take one of your legs and spread her open a bit by isolating one of her legs, pinning it. You have easy access to her vagina and you can guide her hand. Plus, it might help her to relax knowing you can't really see her face while she's touching herself. Once you turn her loose, you have easy access to reassure in her ear how hot seeing her do this is, play with her breasts, nibble her neck.