Is there something going on with the fiance? How is he as a parent? If he is considered a good parent and mom is witholding parenting time then this could be an example of parental alienation. This is where one parent attemtps to influence the child against the other parent.
Your fiance needs to show up for every court ordered visit. If the mom interferes he needs to document it via a police report or perhaps record the conversation. He needs to have a copy of the decree with him everywhere he goes so no one can refute this. Finally, he may need to go in, speak to the principal and convey his concerns. This is a positio where the finace needs to keep his cool, plan accordingly, and plan his case. If he simply follows the rules, documents everything, he may have a good case for custody reversal.