Did he just stain it, or did he stain and seal it? Did he use a combination stain and sealer? Was it water-based? Honestly, let it sit for at least a week outside (direct sunlight will help). If you can still smell even the the slightest hint of odor after that, let it sit for another week. You can definitely use the viv, it just needs time to air out. The length of time will depend on the sealer he used. If he didn't seal it, he definitely should. He can use virtually any water based hard drying top coat sealer. Polycrylic is a popular choice, and cures relatively quickly.