What an awful situation. If he genuinely didn't know about the child then, in all fairness, he cannot be blamed if the relationship was outside of the one he is in now.
Makes me ask why and how the existence of his first child has been made knowledge all of a sudden.
As for your friend Jo, I can appreciate that she must be horrified but , like I have just said, it depends very much upon the circumstances of conception of the other child as to how much blame she can lay at her husband's door.
Hormones will be all over the place, which won't help.
All I can offer sam is that, if he wasn't in a relationship with your friend when his first child was conceived then I feel for him - many people have one night stands and 'get away' with it. If he was in a relationship with Jo at that time then personally, I would castrate him !