As everyone said this can be dangerous, but I think you've got a good start considering you've been friends with this man for 10 years. How long have you and his wife been friends? What kind of relationship do you have with their son? That last is VERY important to think about. Gotta remember it wasn't the kids choice and he may be hurt if things don't go well down the road.
As Red and Schrod said always a good idea to learn more about any situation you're about to become a part of. This forum has MANY wonderful resources, so definitely use them! Also, open and very honest conversations need to happen. Everyone should be on the same page and the only way that can happen is for you all to sit and talk, and keep talking. I think a lot of times people talk at the start of their relationships, but then it just dwindles away for some reason.