You won't have a problem storing cleaned and oiled guns at around 50-55% humidity. A mistake folks make with basements is opening windows or otherwise ventilating them in the summer when outside temperature and humidity is very high. Allowing that hot moist air into a cool basement will cause the moisture to condense on cool surfaces like your guns. I learned this the second summer after I built my house with poured concrete basement walls. I thought a spring opened up because my walls were getting visibly damp near the bottoms and corners. It took a while to figure out but after closing the windows, it became dry as a popcorn fart. A Goldenrod is actually very low wattage. I think the 18" model is only 15 watts. Inside a safe, it will raise the temperature a few degreees and prevent condensation from forming. If your friends' vault is larger than the average safe, a little more wattage will be needed. but it will still be cheaper than running a dehumidifier at full tilt.
