Before you do anything, first find out the speed and rpm that you guys intend to ride at. That is the rpm range of vibration that will be your goal to quell. This is because playing with weights and snakes and lead shot, in rare cases, transforms vibrating bars into smooth enough to live with in a huge rpm range. More common though, is that you are lucky to just be able to address one rpm range of vibration. The weights and snakes and lead shot stuff simply moves the vibes around...moving them to a different rpm range.
Try the easiest first, like I say, pick the ONE primary speed/rpm in top gear, that you want to address. Then borrow some weights from a bike wreckers or friend's bikes, and either size up or size down in weight. This will help tell you what next step to take.
I have also heard that silicone pumped into the bars works, but just like the snake and shot etc, all these measures are easier said than done. There are ramifications and complications no matter which route you try. The silicone can go where you don't want it before it cures (takes a long time to cure because air can't get to the inside part, but slowly) etc etc. If you find a suitably sized bar end weight, that will be your easiest solution. Keep in mind that manufacturers do give this a lot of thought. They do not slap any old weight on thinking..that one looks cool. They are sized/weighted with respect to the anticipated cruising speed of that particular bike, gearing, purchaser demographic etc etc.
Also try some gel type grips, (and gloves) I hear good things about them. Sometimes you have to layer your efforts. And sometimes nothing works well enough to make the bike ridable. I personally have gotten rid of a bike due to the vibration putting my hands to sleep. You only make that mistake once though. I will never again buy a bike that I can't spend a good 30 to 45+ min of sustained cruising at a few different speeds to determine if it's suitable or not. Good luck, I sympathize with her (and you) if nothing works.
