Based on your description (your husband first found the phenotype and his story is complete even without her informatics support), your "ld" should be the first "co-first" author. My suggestions: 1. No delay and this kind of trouble should be solved ASAP. 2. Instead of arguing with the American postdoc, your husband should get his mentor involved. In the conversation with his mentor, he should emphasize that he first discoveries the gene regulation phenotype, which is the core of the manuscript. 3. Tell his mentor that this first "co-first" publication is very critical for his career development. Mentioned the fact that she (American postdoc) agreed to be the second "co-first" author when collaboration begins. Otherwise, he would not agree to combine the manuscript and would prefer to publish separately.