Your husband should file a complaint with the local attorney's association regarding his lawyer's failure to assist, or even provide advice on how to assume custody.
Many lawyers will provide free consultations. He should speak with several to understand what needs to be done. He should be present at that May hearing and have documentation on how his home is the more stable, the schools are better, how he played a big role as a father pre-divorce and post-divorce.
I don't imagine there would be any severe repercussions if your husband just keeps his children home rather than send them to their mother. Chances are his ex doesn't have any money either.

I think you should try keeping a low profile in this situation rather than let his ex know of your involvement.