The good news is that you have a trailblazer and an early adopter. Leverage the Green Belt by sharing the project success widely and broadly. Hopefully others will be interested in learning more and also getting their Green Belt. If there is no Lean Six Sigma infrastructure defined by the organization, then this responsibility would fall to their direct manager.
Hopefully, the Green Belt’s direct manager looks at these skills as an asset, and has discussions about possible Green Belt level improvement opportunities. If leveraging Green Belt skills is not discussed or encouraged by the manager, this could easily become forgotten and then end up being a waste of time, effort and money for the Green Belt as well as the organization.