Hi merrickk and welcome to the forum,
I'm not sure anyone just picks up the guitar and automatically has good rhythm. I think it's just a matter of practice, practice, and more practice. When I was learning the guitar I struggled with rhythm, timing, and everything else a beginner deals with. I could hear in my head what I wanted it to sound like but just couldn't get my picking hand to do it. It seemed like all of a sudden it snapped one day and I was able start keeping better and constant rhythm. I think this hurdle is what keeps a lot of folks from learning the guitar or any other instrument,,,,the basics. So I say, keep on beatin on them strings; she'll get there.
Being a bass player might make this process a little easier since she probably has a good feel for keeping a rhythm. Because she is a bassist, I find it hard to believe she is "rhymthically challenged". It probably seems that way because it is something new to her. That's what I think anyway and don't forget to take advantage of the most friendly, helpful, and knowledgeable people in the world right here on the chordie forum. (that last sentence sounded like a commercial,,didn't it )
"I'm not going to buy my kids an encyclopedia. Let them walk to school like I did." Yogi Berra
"It's like deja vu all over again." Yogi Berra
and my personal favorite Yogi Berra quote: "Ninety percent of this game is half mental."