If your wife's a looker BJJ if she ain't Kickboxing. It’s basically a choice between your wife coming home with increasing amounts of facial damage or her rolling around with a load of muscular swarthy Brazilian guys?!
On a serious note, I know nothing about BJJ, but I wouldn’t say one is easier than the other. You’re talking about chalk and cheese; two totally different approaches to fighting, some people are more suited to one than the other. She should start with the style that she feels drawn to the most to give her a solid base to build on.
If it's not to compete I'd suggest Kickboxing cos it's hugely satisfying to walk out the gym having just kicked the shit out of the pads and bag for 10 rounds, a great work out and a great release. The thing with Kickboxing or Muay Thai is it doesn't take long to learn the basic strikes, so after a few tough months she'll feel empowered and feel like she’s progressing especially if she's the sporty type. As the other fellas have already said what takes the time is putting it all together to create a fluid effective style i.e. knowing when to use those weapons and learning how to comfortably defend against them. But if her goal is to compete that's a whole other thing as you can be a demon on the pads but a mouse in the ring, has she got the spirit, discipline and heart for it? Maybe she does, if so then good luck to her, but it ain’t an “easy” road that’s for sure.