I have several comments:
Magnesium - Another correspondent suggested this. I have heard that magnesium supplementation with Vitamin B6 has been found helpful in preventing recurrence of kidney stones. There are some cautions, however
1) Magnesium is relatively alkaline and will neutralize stomach acid. As a result it interferes with absorption of other nutrients, so, if you take magnesium, stop an hour or two before a meal and don't take more until a couple of hours after a meal. Admittedly, the ideal supplementation window becomes pretty small. Magnesium also attracts water (the chemists call it "hygroscopic.") This tends to increase the amount of water in the user's colon, resulting in a laxative effect. This laxative effect flushes food out before complete absorption of other nutrients.
2) Magnesium supplements - A number of compounds are available. Magnesium hydroxide tablets used to be available in the U.S., but I didn't see any when I last looked. Magnesium hydroxide is the active ingredient in milk of magnesia. The tablets were my favorite form of supplementation, but, as I noted, I can't find them lately. I can find Magnesium oxide, but I think this compound is pretty stable; so it may not release magnesium for absorption very well. Magnesium sulfate is the active ingredient in epsom salts. You've probably deduced that the supplements I like best turn out to be laxatives--which probably entails some concern.
For B6, try 50 mg or so per day. Doses of several hundred milligrams per day for extended periods, unaccompanied by the full spectrum of B complex vitamins, have caused problems. Check the recommended allowance for magnesium. I think it's around 500 milligrams, or so.
Good luck!