Often there isn't much to do, except try to reduce stress on the remaining tree, but it is hard to say without seeing the tree. Are you able to upload a photo of the remaining tree? 

You can also reach out to the local master gardener desk<personal data hidden> or email photos to  <personal data hidden>. 

Thanks for sharing photos. There is no need to fill in the gap. Trees have the ability to seal their wounds and putting anything on or in a wound can inhibit that process. Having said that, though, I would recommend having an arborist come assess the tree soon. Having a wound at the base like that makes the tree vulnerable to failure. The tree is also experiencing new stressors like wind, without the other trunk to dampen wind. 

We recommend working with arborist on the City of Grand Junction licensed tree care providers list: https://www.gjcity.org/DocumentCenter/View/9426/2023-Forestry-License-Tree-Care-Providers-PDF.