Although Tamoxifen resistance does develop in a significant number of women, it does not appear to happen to most women.  As with anything statistical, there is no way to know whether your wife will conform to the majority or set her own pattern.  Because the medicine is working for now, that is what you need to focus on.  Her doctor may be able to make some guesses about how long the Tamoxifen will work, but they will be guesses.  

Do not use any herbal treatments without discussing them with the doctor because some herbs and supplements that might be perfectly safe and even helpful in healthy people can interfere with cancer treatments.

I hope that she gets a long run of success with Tamoxifen.  Many cancer drugs do eventually lose their effectiveness, but when they do, doctors usually have other tricks to try.  The AI's have been very successful, so those may be the next step.