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I am sorry to hear about this situation.
If he does not disclose the situation and the US consular agent finds out anyhow (they do run their own background check), then he will be denied a visa outright for lying under INA section 212(a)(6)(C)(i) (fraud/misrepresentation).
If he does disclose it, then it would be up to the consular agent. The fact that he has not been convicted helps, but also the consular agent may see him wanting to come to US as a way to avoid dealing with the criminal issue. So he would have to assure the consular agent that he intends to go back to take care of the issue. In addition, USCIS generally can deny someone who is convicted for a charge involving turpitude - see HERE. This does involve assault. The fact that he has not been convicted plays in his favor, so it is a "catch 22" in that not being convicted but also having this issue outstanding both helps and hurts him.
Someone in his situation would do well by disclosing, but then also explaining that this was a misunderstanding and which he aims to clear up, and will go back to the Court as needed even from USA, and that his attorney is feeling comfortable that this matter will be dismissed without a conviction.
And yes, he does have to disclose any pending criminal charges as well, I am afraid.
All the best.
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