If the child in question was raised Pagan, it might be a good idea to invite his parents over for dinner and get an idea of what he was taught from them. This will probably make them much more comfortable too, especially if your daughter ends up spending time with them. Remember, the other adults your daughter spends time with raise her too. If he was raised Christian and is now converting to Paganism on his own, don't try to convert him back, and don't encourage your daughter to do so. Many who are rebelling against Christianity will only become sterner in their resolve if people keep bugging them about it. He may come back on his own, and if not, nobody can make him and you'll just make him anti-Christian. If he was raised Pagan (or atheist) and you try to convert him, you'll annoy his parents. If you put yourself in their shoes (or his) you'll know exactly what I'm talking about.
Please check out some of the articles listed in the Articles section of this site. I think they're very good, if I do say so myself and the fact that many other Pagan sites link back to them makes me think so even more. Other than that, I do suggest you talk to the young man, and his parents, and ask them to suggest books for your to read. When you ask someone to explain their religion, you often put them in the difficult spot of not knowing where to begin, so asking for reading suggestions is your best way to go.