Okay, this is probably one of the reasons why her lawyer is trying to settle. Legal Aid won't often represent at trial and will withdraw funding if the case lacks merit.
If I may make a suggestion?
Your son could propose consent orders that make overnights conditional on whatever is necessary to alleviate the risk of abuse.
If it's a mental health condition, then make it conditional on ongoing treatment with dad to receive regular reports from her treating physician.
If it's drug and alcohol abuse, make it conditional on random drug tests.
He could even make it conditional on overnight time being supervised by one of her parents at least, and he could propose that overnight time not start until the kids are older, so they're somewhat more capable of speaking up if mum does something stupid.
And If mum doesn't meet those conditions, then make it a condition that overnight time gets suspended until she does meet them.
If he does go down that path, it would be reasonable to expect such time to occur every second weekend with an afternoon during the week. The Court tends to see merit in the argument that children should have leisurely weekend time with both parents, rather than just one getting all the hard yards during the week, and the other getting all the fun of the weekend.
I don't think there's been much information provided about the nature or frequency of the abuse, nor how it was substantiated, but I'm conscious of the fact there are many cases where allegations of abuse have been made, and so very, very few in comparison which resulted in a finding of same. If the Court doesn’t take the allegations of abuse with as much gusto, then Dad might face criticism from the Court for not being more realistic about the kids time with their mum - after all, these orders will be in place until the kids turn 18, so if Mum takes steps to address the concerns Dad has, why shouldn’t she get a bit more time with the kids?
My view is that it’s always better to be flexible than it is to be rigid about the direction of your family law matter. Being so committed to one outcome that you won’t even consider negotiation otherwise can do more harm than good in the Courtroom.