Your right Jan - weedkiller can be very dangerous, and I don't think you're over-reacting at all. As 'hammerman' says, the kitchen isn't the place for mixing chemicals.
Just read the instructions on the label, which your husband should have done. Although it may be safe when correctly prepared and used, all warnings on the label need to be strictly adhered to.
One more thing, if any weedkiller is left, ensure it's kept in the original container, and not transferred to an old lemonade bottle or similar, if it's a liquid.
I think your husband is showing great disregard for a product that needs to be treated with care, especially if you have children or pets about the place.