He'd been at Friend's house over the weekend and the same sort of thing happened and Mo just left. That didn't have anything to do with coaching. Now he's just doing it very publicly.
This isn't just a situation that involves a coach--it's a situation that involves how his friend's father is treating him.
If this is a long-term friendship between the two boys, then Mo has likely known his friend's father for a long time as well. Has the friend's father ever acted this way toward Mo before?
Has Mo discussed what's going on with his friend? Since it involves his friend's father, and it even goes on when he visits his friend's house, I think Mo should discuss it with his friend, tell his friend how it's affecting him, and ask his friend for advice about how to deal with it, and possibly even ask the friend to talk to his dad about the situation.
I think that preserving the friendship between the two boys is a very important consideration. One way to help preserve that relationship, would be to encourage Mo to share his feelings with his friend. Since it concerns something that involves the friend's father, the friend might have some insight into why his dad is acting this way, and he either might be able to help Mo understand it, or help him handle it, or help him not to get so upset about it, or help to get the situation resolved.
That would also give Mo some way of handling the situation independently, by turning to a friend/peer rather than having you intervene.