My partners ex is stopping him seeing his kids

he is miserable and im helpless to help him. his ex is a mega b***h.she rings him at work and argues with him over them and me and ther past relationship.wen they were 2getha she wood abuse him hit him and acuse him. now that he found me she wnt let him near the kids she told them hes dead.she texs him and uses the kids as a weapon on him. im lost on this as i think he should at least see his kids at ther nanas or somewher without me around.but she wont. wat can i do 4 him?