What exactly is wrong with her hair, other than lack of length? If it's very thin it might be a sign of a medial problem, perhaps a thyroid issue. Wouldn't hurt to check.
If she dyes is, perms it, straightens it, curls it, or anything like that it will be damaged. If she wants long healthy hair she's going to have to stop doing all that. Nutrition is very important to making sure that her hair grows to it's full potential. A healthy diet, good fats, plenty of protein (hair is made of protein), and a good multi-vitamin will help fill the gaps. Go to the health food store or whole foods (I'm biased towards products with fewer chemicals) and get her a deep conditioning treatment. Spend the money and get her a good one. Some people have good luck with egg yolk, olive oil, and a little vinegar. Mix it up and put it in her hair and leave is for an hour before shampooing it out.
I'd steer WAY clear of hair dressers. They are notorious for cutting off more then they are supposed to. If she needs a trim, I recommend going to an old fashioned barber. They know how much a half an inch is and won't try to up-sell her on styles other than a little trim. Some are nervous about cutting ladies hair, but if you explain that you just want it straight across the bottom they do a great job.
The big question is - does your wife want to grow her hair long? You can gently encourage her by complementing her hair a lot and making a nice fuss over any step in the right direction. Tell her how wonderful her neck would look with her hair in a up-do or what ever. I'd skip pointing out other women with hair that you like. No lady liked being compared to women who you might find more attractive. Even if you are just trying to give her an idea of what you like, it's not going to go over well. Focus on how beautiful she is and how wonderful it is that her hair is getting long and pretty.
I hope this helps. Good luck and let us know how it goes!
~Ruby