I know how hard it is to stop someone who is really determined, but the attempt must be made. Best if you try it personally IMO even if it ends up in a confrontation between the two of you. If he listens, great - help him through it. If he goes ahead and does something stupid anyway then at least you have tried, at least you have given him the option to choose a different path. If, after your attempt, you are still friends then you can help pick up the pieces after he has finished with his self-destruction. If you are not still friends then wash your hands of the pair of them.

Either way if the child is in any danger then your local social services should be informed.