Lightning does not turn bones into magnets , these are just superstitions. How long it takes for bones to adapt to the soil depends on the soil pH, the weather and the soil microorganisms or insects. If the pH is high, that is, if the alkalinity of the soil is high, the bones will be absorbed into the soil in just 6 months. If there are more microorganisms or insects in the soil, their effect will be rapid bone loss , in this case it takes an average of two years. In dry climates, bone strength is many times higher, and in the desert, bones remain intact for centuries. You can plant some trees on the grave , then the attraction of the thief will decrease. Or you can bind a grave , from an Islamic point of view, it is not appropriate to bury a grave, but it is appropriate in your context. You can take the advice of the local imam to be sure.