You should then try with helmet[thePlayer] instead of helmet[lp]
EDIT: Try this:
helmet = {}
--lp = getElementType(thePlayer)
if getElementData(thePlayer, 'helmetOn') ~= 1 then
setElementData(thePlayer, 'helmetOn', 1)
local x, y, z = getElementPosition(thePlayer)
helmet[thePlayer] = createObject(2053, x, y, z)
setObjectScale(helmet[thePlayer], 2.4)
exports.bone_attach:attachElementToBone(helmet[thePlayer],thePlayer,1,0,0.03,0.1,0,0,180)
else
setElementData(thePlayer, 'helmetOn', 0)
destroyElement(helmet[thePlayer])
helmet[thePlayer] = nil
end