First, let me highly commend your wife. The WHO recommendation for breastfeeding up to 2yr is not for show. It really can help prevent a whole slew of problems.
Unfortunately, no there are no ways to maintain lactase production in a prolonged lactose free diet. She may get extremely luckily and maintain the bacteria that are needed on her own, but she should not attempt to reintroduce dairy into her diet without the go-ahead from your pediatrician (assuming she's still breastfeeding).
In regards to the second question, can lactase production be regained, my answer is not yet but soon. The textbook and US-FDA approved methods for dealing with lactose intolerance are mostly to avoid dairy and take lactase supplements or tilactase (which will not increase lactase production on their own).
There is, however, a drug currently being studied called RP-G28 (I'm sure they will come up with a nice trade name if it makes it to market). The results were promising. Further, there have also been some promising studies with probiotics, in particular Lactobacillus reuteri, as a possible solution. It has proven difficult to "re-seed" the gut with good probiotics that are swallowed, but the solution may lie with going the other direction.
As a small warning, you have to be very careful when you are shopping for probiotics. It is unfortanitely not well regulated, and the way in which the bacteria are packaged can greatly effect if they are useful or not. For the most part you are going to want live cultures, and if they are reduced to a pill form, you want them lyophilized not heat treated or dehydrated. Many times it can be difficult to get this information from the manufacturer, so please be very careful when shopping. It may be best to ask a medical professional what is available in your markets (including online markets if you have access).
