Best of luck to your wife, it is a very fun sport with a lot of women, won't be long and you will be joining her.
I started competing in the 2011 season and just moved up to an M2. Most of the people I have met use Ruger Vaqueros. A lot of the ladies, prefer the Birdshead models because of the smaller grips. My coach, who dropped back to an L5 this year, has been running a pair of old style Ruger Vaquero Birdsheads for many years. I would suggest the shorter 3.5 inch barrels because they are lighter and easier to point and for a better powder spread to make it easier to hit the ballons. Also, whatever you end up with, buy stainless steel. The stainless will hold up to the black powder better and make cleaning easier, especially when she drops one in the dirt. No matter how hard you try, it will happen and sometimes the only way to clean it is with a waterhose.
The Ruger Vaquero Montado was designed specifically for mounted shooting and the Talo Birdshead "Last Cowboy" Ruger is almost identical. Both use a Super Black Hawk profile hammer and are easier to cock than a regular Vaquero. I started with a pair of Montados with a good action job. Last September I was at Easterns and seriously looked at a pair of the Taylor's Running Irons. In the end I decided against them because the triggers were so light I was afraid I would shoot inadvertently because of the movement of the horse.
Remember, a good holster rig is just as important as a good pair of guns, have your wife look at the various rigs to find what works best for her. Some of the courses have fast gun changes and you don't want your holster to hold you back. I don't know anyone running an off the shelf holster, everyone has custom made mounted shooting holsters. Look through the ads in The Rundown or Western Shooting Horse magazine. I use a Rod Kibbler rig because he is local and although he doesn't compete in mounted shooting he does come to a lot of our shoots. Susan Henning (Dixie Rose Leather) makes a lot of nice mounted shooting holsters as well.