This probably happens more than we would like to see as Lean Six Sigma practitioners. I would say, let them do it. Tell them that if they’ve already found the solution and if there are plans already under way to implement it, then they don’t need DMAIC. This is more of a “Just-Do-It” then a DMAIC project.
In this case, DMAIC would be a waste of time since they’re going to move forward with implementation anyway. Don’t waste their time and if you’re a Lean Six Sigma resource, don’t waste your time!