Sorry to hear that your wife got hurt. I sure hope she comes back for more. Mountain biking is just too much fun! 😀
I second the idea of easier parks as mentioned, and a possible women's ride or clinic. Back when I was a mtbchick, I had the opportunity to teach at women's mtb clinics around the country, and I believe that mountain biking is great for all types of women. Some people just need to take things in baby steps, gaining confidence with each step. Every person has their own risk allowance, and this should always be respected. Make sure your wife only tries what SHE wants to try.
As for the rock gardens: Yes, momentum can be your friend, but confidence, basic skills and a little finesse can get most girls through the same rock garden, and maybe even falling less doing so.
I hope to be organizing some women's skills clinic days in the spring. If I do, I'll make sure to post them up here.